Drabble: The Wonky Faint
josh Moshpit
Title: The Wonky Faint
Summary: Breath-taking Quidditch can really take it out of you. As Hermione knows, life is more than just a game. General, no attributes, one-shot.
There, he could discern just a hint of flickering gold. Without even conscious thought, Harry dropped his broom into an accelerated vertical dive, straight for the Snitch he knew was almost directly beneath him. Malfoy, realizing that Harry had seen the elusive end-game piece, cried out and started diving a split-second later.
Lee Jordan, the ever-observant announcer, began a screaming narrative as the two Seekers pushed their brooms to the speed limits – Harry on his Firebolt, and Malfoy on his Nimbus 2001. Harry could see the Snitch reach a spot mere inches from the grass as they drew closer, when it started scooting erratically in an orbit just above the firm ground.
Harry held his course true, knowing he would have to pull out at the very last moment. Malfoy was hot on his tail, the greater acceleration and top speed of the Firebolt less meaningful since both of them started from almost no velocity at the same moment.
Right as his broom tip was about to brush the tip of the grass on the pitch, Harry pulled up with everything in him, his left hand automatically reaching out to grab the small flying object that ended the game. His joy at the catch was doubled at the muffled thud of Malfoy slamming into the turf as Harry successfully pulled out and flew mere inches above the ground.
Thanks to the unsurpassed extreme broom flying, Gryffindor won the game, the Quidditch Cup, and Harry Potter fell over dead.
What?
Right, we have all read good and bad Quidditch scenes in fanfic and canon. The reality, however, is that the very simple physics of life would kill or seriously wound anyone trying them. I have finally decided to offer a short lesson in the physics of Quidditch and broom flying, in the hopes that people will start putting a tad more reality into the situation.
Let us start with the basics. The Firebolt has an acceleration of 0-150 mph in 10 seconds, according to canon. That translates to an average acceleration of 6.71 m/s/s, which is a good bit less than gravity at roughly 9.81 m/s/s at sea level.
That means, incidentally, that you cannot take off vertically on a Firebolt – in fact, you cannot become "airborne" at all. If you lack acceleration greater than gravity, put wheels on it and call it a magic bicycle, as that is all you will get out of it. However, canon clearly demonstrates that these things fly, so there must be some other "magic" effect here. To ignore the issue completely, I will hand-wave the detail and say that the broom is similar to a wand, and uses the innate magic of the rider to generate a lift force equal to gravity. Now all that acceleration (6.71 m/s/s) can be used to fly at any angle. That means we ignore gravity on the way up, and we conveniently continue to ignore it on the way back down. Newton would be horrified, but it is magic, right?
So far, so good, even if we are sweeping major problems under the "magic" label rug that appears to be bigger than Siberia.
Now, to keep the math and logic simple, assume that Harry is sitting upright on his broom – he is not tilting in any direction (forward, backward, sideways). In essence, he has a perfect seated posture and is sitting on a really thin horse. Other positions will have the same basic result, but they make the maths a tad more complex.
Right. We have our broom, we have our flyer, and we know the broom's acceleration. Frequently, authors seem to assume the Firebolt peaks at 150 mph, and they like to write about flying at "top speed" in these types of dive-for-the-ground, pull-out-at-the-last-moment, catch-the-Snitch, and listen-to-the-idiot-crash scenario that opened this discussion. Really, however, only the acceleration has been framed by canon – the top speed could be anything from 1 mph to Mach 20. In an official Quidditch pitch of 500 feet by 180 feet, travelling at 150 mph, you will completely cross the pitch long-wise in 2.27 seconds, or short-wise in 0.82 seconds. Thus long harrowing chases at top speed simply will not happen. Short harrowing chases might – if you survive them. Very, very short.
If we continue that belief, and assume a constant velocity of 150 mph (67.1 m/s), then we can determine how much g-force is exerted on the flyer by doing a particular trick. Everyone should be familiar with the concept of the g-force, as it is a common issue with pilots, astronauts, roller coasters, race car drivers, and so on.
If the broom flyer had that same perfect posture on the way down in the Wronski Feint, and pulled out at the last second, what would happen?
For our purposes, assume you have a drawing of an upright Harry sitting on a broom in a horizontal direction. During the dive, rotate the paper 90 degrees, to show the dive in perfect posture. As the catch-and-pull-out is performed, slowly rotate the paper back to the original position.
This rotation back to the original position (vertical-to-horizontal) mandates radial acceleration. Generically, a = v*v/r, where r is the radius of the pull-out manoeuvre in our example. Completely ignoring the "extra" effects of gravity as the flyer accelerates toward the ground, since we hold velocity as a constant, we can establish that the radial acceleration at 150 mph is roughly 4.50 km*m/s/s divided by r, the radius of the pull-out curve, where r is measured in meters.
Now, in our scenario, Harry pulled out "at the last moment" where his broom tip was about to brush the ground. Considering a typical broom is around 2m in length, and assuming the pivot point is the centre of the broom, we estimate the radius of the turn as about 1m. That means that the acceleration felt by the flyer (Harry) is approximately 4.5 km/s/s. To put that in g-force terms, divide it by the acceleration of gravity (9.81 m/s/s), and you get a comfortable result of the Seeker going through 459 g-forces.
Is that a lot?
Yeah, he is unequivocally dead. And possibly cut in half by his broom, too.
An aside is required here to discuss g-forces, and what is (and is not) possible with the human body. Did you know that astronauts are generally limited to 2-3 g-forces during take-off or landing manoeuvres? Did you know that the average human can only endure a head-to-toe g-force sensation of 3-6 without significant training? Did you know that fighter pilots can only endure 9-10 g-forces when they are physically trained and using special pressurized flight suits? Moreover, did you know that no one – trained or otherwise – can sustain more than about 3 g-forces inverted (blood rushing to the head) regardless of context?
Of course, these examples are tricky because it is less about the magnitude of the force than the duration of the force. The pressurized flight suit that some military pilots wear (many dislike the thing, as it restricts their ability to move) gives 1-2 g-forces of additional tolerance to the pilot. That means their physical training and breathing exercises get them to handling around 8 g-forces unaided. It is a long way from 8 g-forces to 459.
The human body is pretty remarkable. You can sustain a sudden g-force of up to 200, as long as it lasts in the deep-sub-second region. Such a force for even one second would (supposedly) tear your internal blood vessels to shreds. Car crashes are tricky here, because you have a sudden impact – all those g-forces are applied to a specific location behind the seat belts, and then bones break, and bad things happen. Flight in a well-padded and specially crafted seat is a different matter, distributing the pressure in a meaningful manner. But the higher the g-forces, the less amount of time you can sustain the forces exerted on the body. As little as 2 g-forces, experienced for approximately continuously for 3 seconds, is enough to cause dizziness and vision problems. When the g-forces reach 5, the same duration leads to loss-of-consciousness (LOC) unless you are physically trained and using a pressurized flight suit. The key, however, is that acceleration means a continuing shift in pressure – unchanging values mean no g-force impacts (it becomes the relative frame of reference, much like being in an airplane, automobile, or train).
You can draw a parallel to other parts of life for what g-forces the body tolerates. Sneezing will induce 2-3 g-forces momentarily. Landing straight-legged from a short jump can induce up to 10 g-forces. These clearly fail to cause LOC or visual problems, but the reason is that the exposure time is so short. The blood shift in the body is very brief, as fluid under pressure does not just move around with brief instantaneous forces. The fluid will resist the change, thanks to inertia. Protracted exposure, or extreme magnitude for even brief moments, however, will overpower inertia and the fluid will move. At low g-forces over longer periods, the fluid will pool away from the direction of travel – such as pooling in the feet, where your brain cannot use it. That deprivation leads to vision problems, dizziness, and eventual LOC – see the example of the Formula 1 race at the Texas Motor Speedway in 2001. In the instantaneous variety under very high g-forces, the fluid is moving now and the blood vessels simply cannot withstand the force of movement, thereby tearing open and your internal bits turn into soup. Lovely stuff, that.
But what about being cut in half?
The typical broom is around 0.75 inches thick, but this is Quidditch, so to be generous we will allow the diameter to be in the range of 1-2 inches. Moreover, we will also assume the handle is perfectly round in cross-section, which eliminates stress concentrators that would amplify the force the broom handle would induce on the human sitting atop it. The typical adult male in moderate fitness will be approximately 8 inches through the hips front-to-back, though in a seated posture on a broom the cross-section is likely to be less linear surface in contact. However, being generous again, we will assume a full 8 inches of linear contact over the 1-2 inch diameter broom handle, even though our example is a smaller, younger male. Moreover, let us assume that a skinny teen male weighs a mere 120 lbs.
First, we need to understand how much surface area is being used by the teen. Ignoring second-order effects, we will hand-wave lots of particulars and say that the force is imparted equally across half of the surface area of the cylinder (the bottom half of the broomstick is not in supporting contact with the rider). Thus, we can say that the 8 inch cylinder with a radius of 0.5-1.0 inches comprises a surface area of 25.1-50.3 square inches, only half of which is load-bearing to the rider, so that makes it around 12.6-25.1 square inches of surface.
For a 120 lb human, that translates into about 9.55-4.77 psi (pounds per square inch) of force that the broom applies to the human. Note that this force is being applied to a rather sensitive region of the body. In comparison, using a size 11 (US) shoe on the same male, feet in properly supportive shoes comprise a surface area of about 60 square inches, for a standing-still psi of 2.00.
It is easily established that a broom rider will not be extremely uncomfortable by sitting there and not doing much of anything, so long as the g-forces are set to 1. At the same time, it is not going to be comfortable, either, which is why the Cushioning Charm is incorporated. Try this at home, and you will know just how it feels. However, this entire discussion is a dissection about g-forces, and as the much longer prior analysis pointed out, moving around the pitch at high speed induces g-forces.
Essentially, at 2 g-forces, you weigh twice as much in terms of force applied to your body. So at the 459 g-forces of the dive-and-catch sequence described earlier, the rider's body is experiencing 459 times the normal weight – so from 120 lbs to a rather casual 55,080 lbs. To put that back into psi, using our generous area and load estimates, the rider's pelvic region is being subjected to a measly 4,380 psi on a 1 inch diameter broom, or 2,190 psi on a 2 inch diameter broom. If I were less generous on the surface area of the broom that was load-bearing to the rider, and used a much more aggressive analysis of one-quarter of the cylinder surface area at a mere 6 inches of linear length, then the force would be a pedestrian 11,700 psi for a 1 inch diameter broom, or 5,840 psi for a 2 inch diameter broom, and those numbers are becoming closer to a "real" force load setup compared to the 4,380 psi calculation.
Is that a lot?
It depends on how you look at life, I suppose. A human can bite at 150 psi. A common, generic dog (not mastiff or pitbull or …) will bite at around 250 psi. A macaw parrot will bite at 350 psi, and that is a value that is known to snap a stout non-magical broomstick handle in half. A wolf can bite up to 1,500 psi. A very highly trained martial artist in peak physical condition can impart a kick in the range of 2,000 – 3,000 psi. (Note that such a kick will snap several ribs, break a femur, crack the skull, or the like – whatever is hit with that pressure is breaking.) The to-be-built Freedom Tower, on the site of the former World Trade Center towers, is projected to exert around 14,000 psi on the concrete base. And at the rather extreme end of "common" forces, a .357 magnum bullet impacts at around 40,000 psi. The funny thing about bones is that they tend to be a bit spring-like – slow build-up of force is much easier to handle than a sudden (very short duration) force impact. Breaking ribs requires a snap hit. If you hit-and-push, you are much less likely to break them.
At around 4,380 psi, I think everyone should agree that much pressure in such a sensitive region of the body would induce a faint. A Wonky Faint, even.
Would that flying catch of the Snitch really cut you in half?
Well, if you sustained that pressure (4,380 – 11,700 psi) for any tangible time at all (even as little as one second), yes, it likely would. If instead it lasts a very, very brief moment (perhaps a millisecond or ten), then no, you would just shatter your pelvis and any other bones in primary support positions/alignments. If you were really flying at 150 mph, the duration of your 1 meter radius pull-out would only be 0.04s. That is more than enough to be causing serious damage.
Again, this is a very simple analysis, assumes a perfect circle cross-section of the broom to eliminate stress concentrators, ignores the geometry of the human pelvis, actual load-bearing structure of the human body, and I am not doing force equations for tensile or compressive failure points based on bone density of the average human (femur strength is supposedly in the range of 1.67 * 10^8 N/m/m, knock yourself out to do the maths if you want, but remember the plasticity).
Okay, pulling off one of those spectacular dives and inducing over 400 g-forces is a bad idea. What can the professional flyer do?
Extreme (read: NASCAR) car crashes can exert nearly 200 g-forces (for very, very short moments) on the occupants of the vehicle. NASCAR has quite a collection of data on this, or so I have heard through the years. It is almost certain that the military has equal if not superior data, but the odds of it being public are rather remote. The human body tends to fail fatally when receiving an abrupt full-stop impact of around 80 g-forces. If your body does not stop, but continues moving, then you are less likely to be dead – although you are still very likely to be seriously damaged. It is not the velocity that is the problem, it is the change in acceleration which causes all the problems. This is why you are fine jumping up and down on an airplane, unless it crashes – in which case, jumping up and down will not particularly cause any more damage to you than the crash itself does.
The problem with a car crash is that they tend to leave you in a lot of trouble. United States Air Force physician John Stapp is quite infamous, and worth reading about if you are unfamiliar with the name, but he demonstrated that even as little as 40 g-forces of acceleration coming to a sudden stop will snap bones, crack ribs, and eject dental fillings.
So the idea of a remotely-survivable-but-needing-immediate-medical-aid 200 g-forces acting on the human body would represent the extreme manoeuvring possible. The more mundane 40 g-forces represents a reasonable upper bound of what can be sustained as long as a full-stop impact is not the terminal outcome, if you can pardon the pun. And at 40 g-forces, the pressure exerted by the broom on the rider is in the "manageable" realm of pain, being near a human bite at maximum force. Of course, a magical broom does have that nice Cushioning Charm, which may act as a dampener of sorts to a mere one-tenth of the real force, but even magic would be hard pressed to do more than that. At 200-400 g-forces, any Cushioning Charm effect that prevents serious (shall we say, critical) injury is going to make the Enterprise engineering crew jealous, regardless of the Star Trek generation. It is far more likely to simply state that the Cushioning Charm increases the surface area of a load-bearing contact by an order of magnitude (that means 10x increase, if the term is unfamiliar to you).
Using these two framing points, we can back up and re-visit the radial acceleration. If we assume that we limit our g-forces to be in the range of [200,40], then our radius of pulling out of the constant velocity 150 mph dive is in the range of [2.29,11.5] meters. Remember, that is the radius, not the diameter. The downright pedestrian 3.5 g-forces that a rare "truly scary" rollercoaster produces would translate into a radius of a mere 131 meters. There are supposed to be around 10-20 roller coasters in the world that can induce that much in the way of g-forces. Also bear in mind that a Quidditch pitch is, in the maximum directions, 500 feet by 180 feet, or 152 meters by 54.8 meters.
So if Harry were "tearing about the pitch at top speed, searching for the Snitch", and trying to avoid LOC, then the radius of his turn would be 131 meters. The diameter would be nearly twice the long-wise size of the pitch itself. In other words, unless he has magical binoculars on his face, he would fail to see much of anything at all, particularly with the dizziness and disorientation of a protracted 3.5 g-force flight path.
However, we have done all the work up to this point as a study of manoeuvring at the fanfic cliché top speed and constant velocity of 150 mph (again noting it could be higher or lower, all canon states is the acceleration, and that is not enough to actually achieve flight). But if you back up to the initial paragraph, the two Seekers were flying relatively slow (say, 1 m/s), looking for the Snitch, before they dropped into a dive. The second dose of reality that writers need to get is that you will not reach the "top speed" of 150 mph going from (essentially) rest toward the ground while doing a 100 – 150 ft dive.
What speed would you attain moving from rest into a maximum acceleration downward dive that lasted for 150 feet?
We will again be generous by ignoring air friction and other resisting factors. We will consider the average acceleration from the broom to be 6.71 m/s/s, as previously calculated, but we will ignore gravity's effects. Recall that I stated previously that the broom could not, actually, fly – so to make canon work, I said that magic would counter-act one force of gravity through lift. Thus, the acceleration down is only 6.71 m/s/s, not the 16.5 m/s/s towards the ground were gravity included.
As each second elapses, the distance travelled grows with the square of time. This is elementary physics with a constant acceleration. Note that a height of 150 feet is approximately 45.7 meters. The basic distance calculation is that d equals one-half the acceleration down multiplied by elapsed time squared. So at t=0, d=0. At t=1s, d=3.35m. At t=2s, d=13.4m. At t=3s, d=30.2m, and at t=4s, d=53.7m or about 8 meters underground if the dive started at 150 feet elevation. The actual time to reach the target distance of 45.7 meters is a mere 3.69s.
How fast, in velocity, is the flyer going by the time 2.35s has elapsed at such an acceleration?
About 24.8 m/s, or around 55.4 mph.
Now, we can use this more limiting top flight speed for the dive, and plug it back into our earlier discussion to determine the forces that affect the rider. First, in radial acceleration, if the pull-out is at the last moment for that radius of 1m in the turn, then the rider experiences a simple 62 g-forces. In terms of pressure, this manoeuvre causes 592 psi (1 inch diameter) or 291 psi (2 inch diameter) to be imparted on the rider's pelvis – which we know is rather sensitive.
Obviously, the 1 meter radius absolute-last-moment pull-out is not going to work at this speed either. To drop the g-forces to a more realistic sudden manoeuvre of 10, it requires a turning radius of a short 6.2 meters. The same radius translates to a pressure on the groin of a rather excruciating 95.2 psi – until we remember our Cushioning Charm, which we will hand-wave at an order of magnitude reduction thanks to the increased surface area. Those 10 g-forces would be felt for around 0.39seconds. Not enough to induce LOC, but quite likely to make vision wonky.
So at a full-speed dive (in a vacuum) from 150 ft, with a turning radius of 20.3 feet the manoeuvre becomes survivable without medical aid or self-inflicted castration. It is less than pleasant, and actually attempting to catch something coming out of such a dive will be more than challenging enough, but at least remotely plausible.
Trying to catch anything other than Mr Grim Reaper out of a 459 g-force dive is simply ludicrous.
Interestingly enough, slower brooms than the Firebolt would actually perform better at the Wronski Feint, entirely because the acceleration is less, the final velocity is less, and the sharper turning radius would result in a similar load. Flying a Firebolt at high-speed requires thinking in spherical trajectories, whilst flying an old-school broom would be more elliptical thinking which would allow more direct flight paths. There are plenty of avenues for exploration in the challenges of flying different broom types without completely ignoring the physics of the situation.
The next time you write a broom scene, take a moment to think about the forces involved. The next time you read a broom scene that is absurdly over the top, remind the author that the human body is remarkably resilient – but fails easily under pressure.
Things I spent no time discussing:
Catching a Snitch at 150 mph – whether it is stationary, flying at you, or flying from you. Try rolling down your car window at a mere 60 mph and grabbing a golf ball as you drive past. Assuming you still have intact bones in your hand, think about going more than twice as fast and doing it again. Those g-forces mean your whole body is many times its normal weight everywhere – so fine motor control in the hand, which suddenly weighs 20-200 lbs by itself, while trying to catch the Snitch . . . that is kind of like winning the lottery without purchasing a ticket.
What happens when someone ploughs into the ground at 100-150 mph? The fool following the Wronski Feint is, in essence, ready to be Wet-Dry Vac'd off the field, once they get the sticky mess into one general spot. Since in canon people do seem to survive these little skirmishes with the frustratingly inelastic ground, it is far more likely that the top speeds are "sane" (perhaps 20 mph) during game play rather than ridiculous (90+ mph).
People that know how to drive, fly, or even ride a bike already intuitively know everything in this discussion. You already know to decelerate before taking a turn, to be careful of slip or skid conditions, to compensate for when a turning area is banked or flat, and so on. All I am trying to do is bring that intuition into play for Quidditch, with the reasons why it matters.
Left as an exercise to the reader: dealing with alternate postures of the riders, relative comparisons of brooms, imperfect broom shafts that present stress concentrators, angled dives (rather than vertical), drag forces (air is a liquid, after all), how "unbreakable" is an Unbreakable Charm, non-constant velocity and/or acceleration, and so on. You could probably do an entire thesis work studying Quidditch, the effects on humans, whether you should need a broom-flying license, and all that jive. (Cue Van Halen song here.)
A/N:
I wrote this during lunch, doing many of the equations and such from vague recollections of undergrad – which was a long time ago. I won't claim this is entirely accurate, and welcome anyone that wants to make corrections or even use this as a starting point to launch a proper analysis. This is mostly a rant in the guise of prose, imploring people to think just a tad more.
This article was glanced over by one USMC pilot and (separately) by a neurologist, so while errors are mine alone, at least two people familiar with these issues deemed it sufficient (if somewhat amusing) explanation. An extra credit acknowledgement for a second pair of physics eyes to look it over and catch pesky details goes to Shev. Special thanks to those generous people – you know who you are.
Thanks to a short list: cwarbeck, Chreechree, Sovran, and Sherylyn.
This won't be posted at SIYE (it violates the rules), nor at PSnet. I'll probably stick it on FFnet at some point.
Chris P
I wondered about similar things before, but eventually decided that since they have Cushioning charms (they have to make those brooms comfortable somehow, don't they?) it prevents the fliers from going splat and negates most of the forces that would harm them. This is the magical world, after all, and JKR sucks very much at math.
As for catching the snitch, I don't think the golf-ball analogy is correct. The Seeker is chasing the snitch, so they usually go the same way, at very close speeds - the snitch a bit faster than the seeker - so there wouldn't be a hand-crushing impact when he/she catches it.
Chris
The more people I meet the more I like my cat.
Richard Stringer
I wonder if you could post this in the forums at PS?
R
Green Eyed Devil
dam sham you cant post this at siye evan if it dose violat the one
shot death rule
you know i never evan though of all of this when reading quidditch
sceans ehter in cannon or fanfic but it was very informative ebvan if
i dint understanfd all of it at least i got hte generall jist of it
(it also explans why all the school brooms are so slow it beacuse of
the health & safty implications actully what would a health & safty
inspecter say on a visit to hogwarts & would they eever get any
insurance)
on a seprate note iam fairly sure that the nimbus 2001 & fire bolt
have a sloping front so as they are more areodynamical so ther going
to be evan more painfull to sit on than your example.
James
> – unchanging values mean no g-force impacts (it becomes the relative frame
> of reference, much like being in an airplane, automobile, or train).
>
> You can draw a parallel to other parts of life for what g-forces the body
> tolerates. Sneezing will induce 2-3 g-forces momentarily. Landing
> straight-legged from a short jump can induce up to 10 g-forces. These
> clearly fail to cause LOC or visual problems, but the reason is that the
> exposure time is so short. The blood shift in the body is very brief, as
> fluid under pressure does not just move around with brief instantaneous
> forces. The fluid will resist the change, thanks to inertia. Protracted
> exposure, or extreme magnitude for even brief moments, however, will
> pooling in the feet, where your brain cannot use it. That deprivation leads
> to vision problems, dizziness, and eventual LOC – see the example of the
> Formula 1 race at the Texas Motor Speedway in 2001. In the instantaneous
> and your internal bits turn into soup. Lovely stuff, that.
>
> But what about being cut in half?
>
> The typical broom is around 0.75 inches thick, but this is Quidditch, so to
> be generous we will allow the diameter to be in the range of 1-2
> inches. Moreover,
> we will also assume the handle is perfectly round in cross-section, which
> eliminates stress concentrators that would amplify the force the broom
> handle would induce on the human sitting atop it. The typical adult male in
>
> read more »
josh Moshpit
Richard Stringer
R
SomeGuyFawkes
moshpitstories has PM's, emails and anonymous replies disabled, so I
am posting here.
Triggering post was at: http://www.fanfiction.net/r/4239804/
~-~-~-~-~-~-~-~-~-
What's wrong with my physics? Your's appeared to think that a broom
used it's acceleration for lift. I didn't rag on that because you
later seemed to realize what canon states: Lift is provided by a hover
charm, independent of the broom's horizontal acceleration.
I am a pilot and an engineer and I know exactly how a plane flies.
Specifically how thrust must overcome drag (not gravity) and forward
motion is needed for lift (not a hover charm). I did not give a full
discourse on that because I thought we were on the same page.
A JoeBob summary of broom errors/flight is at:
jbern (period) fanficauthors (period) net/review.php?
sid=342&cid=2463&o=30
(Hope that survives this site's filters.)
Also, that was a summary while you went into more of the derivation.
(Sadly, many FF readers will believe pronouncements but cannot follow
the math. ;-)
Yes, you can use my comments -- I only ask that no deliberate
maliciousness is incorporated.
"I always respond to every review"
I never hold lack of replies against an author. Some might even
resent time replying versus posting chapters but I am grateful for
whatever we get.
Thanks for your very good stories.
> "Howdy.
>
> I'd really like to respond to your comment publicly, as it shows a very
> common misconception about how planes really fly. The physics you're
> trying to use don't work for a lot of reasons.
>
> The problem is two fold. One, I don't know if the source of the error
> is yours or the prior work you're mentioning (JoeBob?). So, I'd
> appreciate a pointer to whatever that was about. Two, I'd like your
> approval to use your comment as the basis for a second "chapter" as it
> were to the physics article.
>
>
> The first is generally useful to me, as it helps me separate what your
> comments are from the source article. The second is important to me
> because you're a fairly decent reviewer on the stories, which is rare,
> and the only way I can use your comments as a basis will wind up risking
> you reacting negatively. I won't be making your comments out to be
> idiotic or anything -- far from it, it's a common misunderstanding. But
> the correction of that misunderstanding does have connotations, and no
> matter how non-confrontational I try to do it, it's bound to seem that
> way. I can do it without using your comments directly, but it makes it
> a bit more complex to set properly.
>
> So, how would you feel about that?
>
> Thanks for the reviews, btw. I always respond to every review (well,
> those not at FFnet). I've just been so slammed I've let it all slide
> for a while. I'm only now coming back up to speed with too much non-
> work-related material that's been backlogged.
>
> Cheers."
Kevin
>
> Title: The Wonky Faint
>
> Summary: Breath-taking Quidditch can really take it out of you. As
> Hermione knows, life is more than just a game. General, no
> attributes, one-shot.
>
That was hilarious, and I'm glad you finally got around to writing that
up and sharing it.
Please, please, PLEASE sent that to JKR. :-)
I know this was all about acceleration's effects on the body, but I'd
love to see another section on Beaters. Aren't bludgers described as 8
inch iron balls? And beater bats as having an unbreakable charm on them?
So what happens when they hit a person, or heaven forbid, a seeker on a
chase runs into one on a dive? (Hmm, going fast and hitting a small and
extremely hard object? Sounds worse than the catching a golf ball at 150
mph that you mentioned, or at least it does to me. Didn't Harry get hit
in the head by one of these things on canon?)
I'm glad you referenced the infamous "inertial dampners" from the Star
Trek world. I guess brooms must have that kind of charm built into them.
I've also found the long chase scenes from the movies to be ludicrous.
Those scenes have at least 1 other big problem, but I'll just assume
that's movie madness.
I think we all write those crazy not really possible Quidditch scenes
because: 1) JKR did it; 2) they're fun. I know I have. I'll try to
lessen them, but hey, the HP world has magic. I've always viewed magic
as a way to overcome physical limitations (or the physical Newtonian laws).
Kevin
Malchior
it's fantasy and anything you want can happen in fantasy. The bludgers
move fast enough to break an arm on contact, I seriously believe they
could quite easily kill someone (especially when looking at the movie
versions).
I think it must be mentioned that JKR said wizards are far tougher
than muggles are. Maybe it has something to do with their magic? I
really never could figure out why the seeker, the person who floats
around on a broom until the snitch is spotted, is so tired after a
practice session. Maybe the magic of the player is in constant use
countering all the g-forces that are in play during Quidditch. If Ron
can be beat in the side of the head by a giant chess piece and be
fine, I'm sure wizards could deal with something as 'trivial' as g-
forces.
squidtamer
I read through this. Think I'd see one math issue, but it's so late
for me, it could be my brain asleep.
Sent the link to a buddy over at NASA for a laugh I'm sure he'll
mention any real math problems if you really want. Sorry, been in the
service forever so this was actually something we joked about when
some of the pilots' kids (and some of the pilots as well) began to
read and enjoy the early HP books.
It always struck me that JKR's answer to anything remotely unworkable
was either "oh dear, the maths" or "It's magic, of course!" so why
not acceleration damping, mass reductive, blood flow restrictive,
physics canceling spells, etc? *shrug*
I think one of my favorites fanfic examples, and quite honestly I
forget where its from, Harry does a top-speed run into a goal hoop and
proceeds to 'hook a foot' and use it for an Immelman turn. I had a
good laugh about it at the time.
But after all, it's all in fun, isn't it? So why push the issue on
the non-technical HPFF world?
Otherwise I'd have written a treatise on why the second task in book 4
is complete rubbish due to depth, time, and DCS. Hey, it's what I
teach/do for a living, so sue me.
ST
-it's not the fall that kills you. It's that sudden stop at the end.
Kevin
> ...
> It always struck me that JKR's answer to anything remotely unworkable
> was either "oh dear, the maths" or "It's magic, of course!" so why
> not acceleration damping, mass reductive, blood flow restrictive,
> physics canceling spells, etc? *shrug*
>
"Magic" is a great answer to a lot of problems. ;-)
> I think one of my favorites fanfic examples, and quite honestly I
> forget where its from, Harry does a top-speed run into a goal hoop and
> proceeds to 'hook a foot' and use it for an Immelman turn. I had a
> good laugh about it at the time.
>
That would be "Hope" by Jeconais. He did point out in the story that if
done wrong, you'd lose your foot. I'm with you, in that I think a person
would lose their foot no matter what if it was done at more than a slow
speed.
> But after all, it's all in fun, isn't it? So why push the issue on
> the non-technical HPFF world?
>
> Otherwise I'd have written a treatise on why the second task in book 4
> is complete rubbish due to depth, time, and DCS. Hey, it's what I
> teach/do for a living, so sue me.
>
As long as they kept to 40ft or less, would it really be a problem? Of
course, you should rightly tell me that any lake that big will easily be
more than 40ft in the center. :-)
> ST
>
> -it's not the fall that kills you. It's that sudden stop at the end
>
Arresto Momentum! :-)
Kevin
Alex Austin
> ...
>
> Let us start with the basics. The Firebolt has an acceleration of 0-150 mph
> in 10 seconds, according to canon. That translates to an average
> acceleration of 6.71 m/s/s, which is a good bit less than gravity at roughly
> 9.81 m/s/s at sea level.
Keep in mind, though, that acceleration/thrust isn't necessarily
constant. I can certainly imagine that the Firebolt can accelerate at
15-20m/s/s from a stop, but that the acceleration goes down with
speed, allowing the speed to asymptotically approach 150mph. So it may
reach 150mph in 10 seconds, but it'll reach 120 in 5. Such a "thrust
curve" would make the broom excellent for sharp maneuvers and okay for
long stretches.
> ...
> Right. We have our broom, we have our flyer, and we know the broom's
> acceleration. Frequently, authors seem to assume the Firebolt peaks at 150
> mph, and they like to write about flying at "top speed" in these types of
> dive-for-the-ground, pull-out-at-the-last-moment, catch-the-Snitch, and
> listen-to-the-idiot-crash scenario that opened this discussion. Really,
> however, only the acceleration has been framed by canon – the top speed
> could be anything from 1 mph to Mach 20. In an official Quidditch pitch of
> 500 feet by 180 feet, travelling at 150 mph, you will completely cross the
> pitch long-wise in 2.27 seconds, or short-wise in 0.82 seconds. Thus long
> harrowing chases at top speed simply will not happen. Short harrowing
> chases might – if you survive them. Very, very short.
I'll concede that top speed is not proper. I autocross. While driving
on the course is quite exciting, many cars, even sports cars won't
reach 45mph on a typical autocross course because you spend too much
time accelerating, decelerating, and turning to build up any speed.
So, I'd say that a long harrowing chase would involve a top speed
around 45-60 mph, and that it would involve a lot of sharp turns.
>
> If we continue that belief, and assume a constant velocity of 150 mph (67.1
> m/s), then we can determine how much g-force is exerted on the flyer by
> doing a particular trick. Everyone should be familiar with the concept of
> the g-force, as it is a common issue with pilots, astronauts, roller
> coasters, race car drivers, and so on.
>
> If the broom flyer had that same perfect posture on the way down in the
> Wronski Feint, and pulled out at the last second, what would happen?
You would hit the ground. On the other hand, if you accelerated
straight down for as long as you could and pulled up at the last
second before you knew you would crash (1/2 height from where you
started at no velocity), you would then have thrust upwards, slowing
your speed so you have a hyperbola with the apex near the ground and
snitch so that you can catch it (while going ~5mph or less).
> ...
> But what about being cut in half?
> ...
>
> Of course, a magical broom does
> have that nice Cushioning Charm, which may act as a dampener of sorts to a
> mere one-tenth of the real force, but even magic would be hard pressed to do
> more than that.
I've actually imagined that the Cushioning Charm is a bit more like a
constant height Wingardium Leviosa - it lifts every bit of mass in the
rider by the same amount, keeping them just barely in contact with the
broom.
> Using these two framing points, we can back up and re-visit the radial
> acceleration. If we assume that we limit our g-forces to be in the range of
> [200,40], then our radius of pulling out of the constant velocity 150 mph
> dive is in the range of [2.29,11.5] meters. Remember, that is the radius,
> not the diameter. The downright pedestrian 3.5 g-forces that a rare "truly
> scary" rollercoaster produces would translate into a radius of a mere 131
> meters. There are supposed to be around 10-20 roller coasters in the world
> that can induce that much in the way of g-forces. Also bear in mind that a
> Quidditch pitch is, in the maximum directions, 500 feet by 180 feet, or 152
> meters by 54.8 meters.
>
> So if Harry were "tearing about the pitch at top speed, searching for the
> Snitch", and trying to avoid LOC, then the radius of his turn would be 131
> meters. The diameter would be nearly twice the long-wise size of the pitch
> itself. In other words, unless he has magical binoculars on his face, he
> would fail to see much of anything at all, particularly with the dizziness
> and disorientation of a protracted 3.5 g-force flight path.
>
Once again, we agree that "top speed" is an unlikely situation.
> ...
> We will again be generous by ignoring air friction and other resisting
> factors. We will consider the average acceleration from the broom to be
> 6.71 m/s/s, as previously calculated, but we will ignore gravity's effects.
> Recall that I stated previously that the broom could not, actually, fly – so
> to make canon work, I said that magic would counter-act one force of gravity
> through lift. Thus, the acceleration down is only 6.71 m/s/s, not the 16.5
> m/s/s towards the ground were gravity included.
I figure that on a professional broom, the hover charm automatically
reduces/disables itself when you attempt to dive, allowing you to dive
faster, but also requiring that you pull out of the dive much sooner.
> ...
> Interestingly enough, slower brooms than the Firebolt would actually perform
> better at the Wronski Feint, entirely because the acceleration is less, the
> final velocity is less, and the sharper turning radius would result in a
> similar load. Flying a Firebolt at high-speed requires thinking in
> spherical trajectories, whilst flying an old-school broom would be more
> elliptical thinking which would allow more direct flight paths. There are
> plenty of avenues for exploration in the challenges of flying different
> broom types without completely ignoring the physics of the situation.
Thus the difference between a professional broom (like a Firebolt) and
a pedestrian broom (like a Cleansweep 4). The professional broom has
lots of (what I call) overhead so that the rider can push it to one
limit at a time, rather than all limits simultaneously.
An example would be the dive I described earlier involving turning
around and pointing up when halfway down to the ground. Of course it
wouldn't be halfway if the hover charm released when diving.
>
> The next time you write a broom scene, take a moment to think about the
> forces involved. The next time you read a broom scene that is absurdly over
> the top, remind the author that the human body is remarkably resilient – but
> fails easily under pressure.
I most definitely will keep this discussion in mind when I attempt to
write broom scenes. I probably would've avoided the phrase "top speed"
by myself, but it's a nice reminder to actually put some numbers into
this discussion.
>
> Things I spent no time discussing:
>
>...
>
> What happens when someone ploughs into the ground at 100-150 mph? The fool
> following the Wronski Feint is, in essence, ready to be Wet-Dry Vac'd off
> the field, once they get the sticky mess into one general spot.
Poor Draco... Of course most authors (Rowling included) seem to
portray him as someone the world is better off without, so...
> People that know how to drive, fly, or even ride a bike already intuitively
> know everything in this discussion. You already know to decelerate before
> taking a turn, to be careful of slip or skid conditions, to compensate for
> when a turning area is banked or flat, and so on. All I am trying to do is
> bring that intuition into play for Quidditch, with the reasons why it
> matters.
Thank you for the discussion.
- Alex
josh Moshpit
This is a reply to a review reply from fanfiction.net. However,
moshpitstories has PM's, emails and anonymous replies disabled, so I
am posting here.
Yeah, that's a side effect of the DLP zealots and jihadlings.
What's wrong with my physics?
I think the key point is going to be summed up as follows: two people that understand the physics and aren't fully communicating, leading to confusion.
This is what I was aiming for, based on your FFnet comment:
"Acceleration less than one G means you cannot become airborne." WRONG! Almost every airplane ever made has thrust acceleration less than one G. The only exceptions are SOME jet fighters and rocket-planes. These craft fly without magic, just fine.
Now, set aside your knowledge of what you know, and take a look at it from my point-of-view (before I knew you were a pilot). In the absence of any other information, this seems like a misleading statement -- there's no mention made of lift, which I did refer to very early on, that is required if your acceleration isn't greater than gravity. That's my point about the brooms, the accel isn't enough to get it flying unless there's some other factor contributing -- which is where I drew the analogy of a wand and using the rider's magic to get the lift (hover charm) which counters gravity.
Based on comments you made later, it seems we're in agreement on this, just not communicating on the same page. (I'm not an aerospace guy, btw -- I used to do solid-state stuff.) There's always the possibility that I'm flat-out wrong on things as well, to which I'm always open to corrections on.
Your's appeared to think that a broom used it's acceleration for lift. I didn't rag on that because you later seemed to realize what canon states: Lift is provided by a hover
charm, independent of the broom's horizontal acceleration.
This is probably one of those spots that would be better re-worded in any future additions, but the point I was after is that without a lifting force of some kind, nothing was happening. It wasn't a later realization as much as it was pointing out that a lift force (hover charm) is required, which is what I put in that "magic bicycle" bit at the very beginning. Mostly, I wanted to get that topic out of the way, and just focus on the g-force question under very simple assumptions.
The JoeBob analysis sounds interesting. I'll have to go take a look at it later today, just to see how the analysis was carried out. The URL got munged pretty thoroughly, but I'll reconstruct it. Thanks for the pointer.
-josh
josh Moshpit
Kind of a lump catch-up on comments here...
Alex: One of the things I really was after was to keep all the assumptions very simple and easy to follow. Having had the (mis-)fortune of guest teaching a few high school physics classes, I wanted it all to be as clear as possible for implications -- even with ridiculous assumptions. But I agree entirely. Although, if I could get a car engine that delivered constant torque across all RPMs . . . (I made lots of simplifications. Constant velocity? Ignoring gravity with the hover charm? Uniform acceleration? Discounting drag? Of course, I pointed out all these simplifications at the end, but . . . ) Anyway, your key comment is the one I was after -- people are not going to tear around the pitch at 60+ mph. A more plausible game-speed is in the 25-45mph range. And that's more than enough to be carrying on with.
Lots of people: (paraphrasing) "It's magic. It's not worth getting worked up over." Yeah, yeah. The problem is that there are only so many times I can stand to read about those "top speed" (some even identifying it by mph), diving for the ground, pulling out at the last second things before I want to shake people. (Goes back to my anger management issues, right?) As long as people pause, think about it, and at least make an effort to reconsider what they're writing about a bit, that's good enough. It's not like the end result will be all that much different.
A few people: (paraphrasing) "Wizards are tougher than Muggles!" Commonly cited in Neville-the-bouncing-Squib bit. I completely discount Neville's incident, as he was dropped out of a window (maybe 14-30 ft off the ground) which is fully survivable, but his accidental magic flared and he bounced away. That was his demonstration that he wasn't a squib. Of course, if he were a squib, he'd have been rather hurt, but that's another story. As for Wizards being tougher than Muggles, I ask this - define "tougher". Just think about the blood in your arteries and veins for a moment, and ignore the rest. The g-forces are going to apply your blood regardless of how "tough" you are. Hitting 459 g-forces is going to apply so much force to your blood vessels as a Muggle that your arteries/veins will simply shred, let alone organs and the like. If you're magical, just how much tougher _are_ your blood vessels? And what are the implications there? "Tougher" blood vessels means less flexibility which directly causes higher blood pressure, and earlier death. Much like the other points in the write-up, it's a question of how much hand-waving you're willing to do, and what the constants are. 1 Magical Person = k * Muggle person. What's k? 1.5? 3.14159? 10?
Anyway, despite the gross simplifying assumptions (which let me work the calculations out on a scrap of paper during lunch), the objective was to make people think. As long as people are thinking at least a little more about it, I'll consider it a success. And if people are asking questions (like they have been), then it's even better. (And bludgers, which I completely ignored...)
Cheers,
-j
ST: Yeah, the second task was an amusing issue. I was muttering about dive times and what not during the whole thing. The really funny part was when my wife tossed out a casual comment about it, since she's an advanced science diver and had to be very, very careful in some of what she used to do. Probably not as edge-pushing as what you did, though.
SomeGuyFawkes
> Based on comments you made later, it seems we're in agreement on this, just
> not communicating on the same page. (I'm not an aerospace guy, btw -- I
> used to do solid-state stuff.) There's always the possibility that I'm
> flat-out wrong on things as well, to which I'm always open to corrections
> on.
days later, I see that I could have been more clear. I apologize for
letting my annoyance show through.
Anyway, your calculations and analysis were correct with the small
exceptions that I attempted to point out. To wit:
a) It appeared that you thought lift depended on thrust -- which is
not true for either brooms or most aircraft until second-order effects
are considered.
b) Analysis of the broom diameter should have come only after it was
plain that a cushioning charm normally made it irrelevant.
c) Average acceleration over 10 seconds (an eternity in flight,
boxing, and bull-riding) is no indication of peak acceleration.
I'll stop nit-picking since we appear to be in accordance.
Some other things you might consider:
1) Wind, especially in storms, will have big effects. The winds, say,
50 feet up are substantially stronger than those at 6 feet and often
in different directions.
2) Bird and/or insect strikes.
3) Jo seems to place special importance on the twigs and suggests that
the bulk of thrust charms are there. If that is true, all kinds of
nasty torque effects come into play. She talks about streamlining
the twigs -- which is absurd considering the other drag-inducing
components of the system.
4) The actors have complained how uncomfortable a broom is in tame
studio conditions (admittedly without a cushioning charm).
5) Motion sickness, altitude sickness, and popping ears.
6) On cross country,night, and/or poor-visibility flights, there are a
slew of navigational and pilot illusions. These kill real people all
the time (Ala JFK Jr.). Do they put ILS receivers on
brooms? ;-) Note that craft with hover capabilities (helicopters,
brooms, Harriers) are not immune.
Good thing we don't read Jo for her physics, math(s) or realistic time-
lines. :-)
calgary
noticed on the last movie, that there are foot rest pedals on the
brooms. I don't recall ever seeing that before. Was is described in
the books?
SomeGuyFawkes
On May 12, 7:03 pm, calgary <rachelr...@gmail.com> wrote:
> Just a quick question about the brooms shown in the movies. I just
> noticed on the last movie, that there are foot rest pedals on the
> brooms. I don't recall ever seeing that before. Was is described in
> the books?
>
These are more differences between practical and scriptural brooms.
josh Moshpit
Richard Stringer
mind you, I supposed they don't get helmets too ;-)
R
Brisk
lessen how much g-force the rider feels? Or some sort of shield
against the wind? The books never went really in depth on what
exactly was on a broom but I'm positive that one would have more
advanced charms than simple control and cushion charms. But even with
that there should still be limits. How much would an anti-gravity
charm reduce from 459gs? Probably not enough for the rider to survive
because despite it being a magical world, 459 is still a relatively
enormous number.
The fact is, the majority of fanfic writers don't catch small (and
admittedly some not-so-small) details like these. It's the kind of
attention that would separate good writers from great writers given
that all other factors (plot/characterization/delivery) were the same
in terms of quality.
SomeGuyFawkes
On May 13, 5:39 pm, Brisk <Brisk...@gmail.com> wrote:
> What's to say there aren't any anti-gravity charms on the broom that
> lessen how much g-force the rider feels? Or some sort of shield
> against the wind? The books never went really in depth on what
> exactly was on a broom but I'm positive that one would have more
> advanced charms than simple control and cushion charms.
streamlining twigs, aerodynamic lines and polish, etc. Further, such
a charm would block or lesson the impact of rain -- which does not
happen.
"G-force lessoning" charms would make things like Harry's bucking
broom a non-problem. Further, VFR pilots often fly by the "seat of
their pants"** -- that is, by the inertial changes felt. This kind
of flying is enjoyable and would likely be a desired part of a
quidditch experience. So, a gaming broom would be least likely to
have such a charm -- just like sports cars usually have firmer
suspensions.
Also, Jo goes into length with lame charms like "braking" -- which
would be a prerequisite to ANY safe broom. Yet she does not have
even a sticking charm to keep the rider attached.
We know JKR sucks at math, physics, and certain types of logic***.
She also seems to have no flying/gliding experience apart from the
usual cattle-car ride. So Occam's Razor suggests that Jo just didn't
think it through -- concentrating on the main story, bless her.
** "Seat of the pants" is deadly for non-sport flying however. Point-
to-point pilots rely on their mind and instruments or eventually die
or limit themselves to clear, sunny, daylight -- over land only.
*** Further, Jo admits this. Hence she calls these "children's'
books" (read: "logic and real-life details do not apply"). And, she
sets them in a magic world (read: "I make the rules, your physics
mean nothing").
Pfeilspitze "Mercredi" Armbrust
>
> So Occam's Razor suggests that Jo just didn't
> think it through -- concentrating on the main story, bless her.
>
Except that she didn't really think the story through, either...
Richard Stringer
A large tower, nearly a hundred feet tall, rose out of the air, and Harry recognized it by the cross on top of it as a church tower. He dove past it, but grabbed the cross, wrapping an arm around Ginny and pulling her with him. The broomstick continued to zoom away, and Voldemort, who had not seen Harry's escape from his broom, continued to chase it. Harry, meanwhile, swung from the cross and landed on the rooftop, holding onto Ginny with his other arm.
A simple manoeuvre, yes?
SomeGuyFawkes
On May 16, 3:04 am, "Richard Stringer" <parakle...@gmail.com> wrote:
> I was checking through a few stories on ff.net and came across this....
>
> but grabbed the cross, wrapping an arm around Ginny and pulling her with
> him. The broomstick continued to zoom away, and Voldemort, who had not seen
> Harry's escape from his broom, continued to chase it. Harry, meanwhile,
> swung from the cross and landed on the rooftop, holding onto Ginny with his
>
> A simple manoeuvre, yes?
>
Never mind Spider!Harry™ (Now with detachable arms!). How about that
floating tower, rising from air?
Phil Boswell
> On May 13, 5:39 pm, Brisk <Brisk...@gmail.com> wrote:
> > What's to say there aren't any anti-gravity charms on the broom that
> > lessen how much g-force the rider feels?
> broom a non-problem.
those charms? We're never told exactly what it was he managed to do,
just that it was supposed to be well-nigh impossible.
Wasn't part of the trouble with checking the Firebolt that there were
so many interlocking and overlapping charms that Flitwick and
McGonagall had to go to huge lengths not to interfere with them?
--
Phil
SomeGuyFawkes
Rowling covered her arse with vagueness -- a standard and despicable
ploy.
I have found that with criminals, politicians, and zero-point energy
nuts, if they do not mention the details, it is because they do not
know them or they are unfavorable.
I bet that if we ask JKR *what* those charms were, she would not list
anything like "inertial dampening" charms.
calgary
minute ago. Yeah..I burst out laughing because I had just seen this
post. . The adventures of psyco Harry & Ginny. Voldemort seems
almost normal. Don't know how sociopath Harry will turn out, but
that flying sequence was too much. Hope he writes some more.
On May 16, 6:04 am, "Richard Stringer" <parakle...@gmail.com> wrote:
> I was checking through a few stories on ff.net and came across this....
>
> but grabbed the cross, wrapping an arm around Ginny and pulling her with
> him. The broomstick continued to zoom away, and Voldemort, who had not seen
> Harry's escape from his broom, continued to chase it. Harry, meanwhile,
> swung from the cross and landed on the rooftop, holding onto Ginny with his
>
> A simple manoeuvre, yes?
>
> 2008/5/14 Pfeilspitze Mercredi Armbrust <pfeilspi...@gmail.com>:
>
>
>
>
> > On Tue, May 13, 2008 at 9:14 PM, SomeGuyFawkes <w0off...@sonic.net> wrote:
>
> > > So Occam's Razor suggests that Jo just didn't
> > > think it through -- concentrating on the main story, bless her.
>
>
> - Show quoted text -
Malchior
think of them taking every spell and charm off the Firebolt and then
reapplying them. More likely, they sent it back to the company for the
reapplication.