Physics of recoil

[Simon Quellen Field]

Physics doesn't care what makes sense to you.

:-)

Kinetic energy is one half the mass times velocity squared.

If the rifle doesn't move, it has no kinetic energy.

If it only moves slowly, then it has little kinetic energy compared to the

high velocity bullet. The ratio of the half masses is low compared to the ratio

of the squared velocities.

By your intuition, a ball that hits the ground and bounces back up would be

giving half it's energy to the sidewalk.

In your second intuition failure, you are confusing inelastic collisions with

elastic collisions. The energy of a bullet hitting something like an armoured

box or a car will be mostly spent damaging the box and the bullet, generating

heat. That is an inelastic collision. The same amount of energy applied in a

slower collision, say one BSC colliding with another, will cause the second one

to move much like a cue ball causes the eight ball to move.

Before replying with more intuition, consider reading about the subject:

" http://en.wikipedia.org/wiki/Recoil "

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On Mon, Mar 5, 2012 at 9:45 AM, Phil <phil...@gmail.com> wrote:

> Well, no.

> In the case of a 2.25 kg pound rifle firing a 9.7 gram bullet, less than
> half a
> percent of the energy goes into the rifle, and more than 99.5% goes into the
> bullet.

You seem to be saying that the fraction of energy going into the
bullet is equal to the fraction of total mass made up by the rifle.
This makes no sense to me.
An explosion between the bullet and the rifle should transfer about as
much energy into the bullet as into the rifle.

A person in body armor can be shot with a shotgun at close range and
not knocked down; and the kick from firing a shotgun is enough to
knock someone down who isn't braced for it; so the magnitude of these
forces must be similar.

>Let's say it came to rest in a single second, so the speed is 0.1 meters
per second.

No, a second is a very long time for these things.  I would be
surprised if the speed was less than 10 meters per second.  If you
knew the coefficient of friction you could compute the minimum force
needed to make the thing start moving and the speed at which it would
move.

Put it this way:  If you fired your 420j .38 into an armoured BSC, I'd
be very very surprised if it moved at all.  So it does not make sense
to say it moved 10cm after absorbing 3 joules.

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[Daniel C.]

On Mon, Mar 5, 2012 at 1:46 PM, Simon Quellen Field <sfi...@scitoys.com> wrote:
> Before replying with more intuition, consider reading about the subject:
> " http://en.wikipedia.org/wiki/Recoil "

Fascinating article, which I'll finish when I'm done with homework for
the day. In the meantime, could you comment on the "slow, heavy" vs.
"fast, small" projectile debate that's pretty much never ending among
gun nuts?

Thanks,
Dan

[John Griessen]

On 03/05/2012 01:00 PM, Daniel C. wrote:
> could you comment on the "slow, heavy" vs.
> "fast, small" projectile debate that's pretty much never ending among
> gun nuts?

They're concerned with loss of KE through air friction. So, some slow heavy
leaning is good if you shoot long distances, but not too much, or more drop,
more travel time in which to miss. Think about air friction in the
"fast, small" projectile case.

Air friction can be good when designing stirred air heaters, incubators,
to get back to the bio topic.

[Phil]

On Mar 5, 1:46 pm, Simon Quellen Field <sfi...@scitoys.com> wrote:
> Physics doesn't care what makes sense to you.
> :-)

>...

> Before replying with more intuition, consider reading about the subject:
> "http://en.wikipedia.org/wiki/Recoil"

Before replying with more snarky comments,
describe the calculation you did to conclude that 99.5%
of the energy goes into the bullet.

[John Griessen]

On 03/07/2012 10:07 AM, Phil wrote:
> describe the calculation you did to conclude that 99.5%
> of the energy goes into the bullet.

The balance in a bullet launch is
that both the gun, and the bullet get accelerated the same amount of time.
Simon probably just did a rough guess based on the KE formula and that
it has a V**2 term for each, gun mas is 1000 gram, bullet mass is 25 gram,
and one's V is .1 meters/sec if the gun is
constrained from free moving, and the bullet is maybe 600 m/s, so the ratio
of energies mv**2/2 is 10:9M

Giving it half a percent was conservative and allowing lots of absorbed energy loss.

[Simon Quellen Field]

My apologies if my attempt at humor seemed snarky to you.

The smiley face was meant to convey a friendly smile.

The calculation was actually in the article I asked you to read before posting

any more thoughts on the matter. I rounded the number, which is actually 99.57%,

so I was actually being a bit conservative:

The mass of the firearm () is generally much greater than the projectile mass () which means that most of the kinetic energy produced by the firing of the firearm is given to the projectile. For example, a rifle weighing 5 pounds firing a 150 grain bullet, the recoil energy will be only 0.43 percent of the total kinetic energy developed. In the case of zero-recoil, the firearm will gain no energy, and the energy of the projectile will be increased by 0.43 percent over that of the free-recoil case. 

A clearer way to state the last sentence was my example of resting the rifle butt on

the ground and firing into the air. "The firearm will gain no energy" means that 100%

of the energy goes into the bullet and the escaping gases. Adding 0.43% to 99.57%

gives 100%, which the article could have simply stated, making it much easier to follow.

This is fairly simple physics, and fairly simple math, but this is a DIY biology thread,

and we have probably exhausted most of the interest of the group in this side-thread.

I changed the subject so people could more easily kill it if they are bored. I often

describe my job as teaching physics to people with short attention spans, and I am

afraid I may have gotten carried away this time.

:-)

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[Nathan McCorkle]

On Wed, Mar 7, 2012 at 12:35 PM, Simon Quellen Field <sfi...@scitoys.com> wrote:

my example of resting the rifle butt on

the ground and firing into the air. "The firearm will gain no energy" means that 100%

of the energy goes into the bullet and the escaping gases. 

Doesn't the ground feel the force from the explosion that is equal to the force applied to the projectile? Isn't that Newton's "for every action there is an equal and opposite reaction"?

 

--
Nathan McCorkle
Rochester Institute of Technology
College of Science, Biotechnology/Bioinformatics

[Simon Quellen Field]

Did any of you take any physics classes in college?

:-)

Force is not energy.

You can push as hard as you like on the wall, but if it doesn't move, you

haven't transferred any energy or done any work.

A brick puts force on the ground just by sitting there.

But as long as it is just sitting, no energy transfer is taking place.

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[Nathan McCorkle]

On Wed, Mar 7, 2012 at 7:35 PM, Simon Quellen Field <sfi...@scitoys.com> wrote:
> Did any of you take any physics classes in college?
> :-)
>

Nope, its at the top of my list for Community College after I graduate
from RIT in a few months.

[Daniel C.]

On Wed, Mar 7, 2012 at 7:35 PM, Simon Quellen Field <sfi...@scitoys.com> wrote:
> Did any of you take any physics classes in college?

Neither in high school, nor in college. Your question reminds me of
something I kept wondering about my fellow soldiers when I was in
Infantry school - "Didn't any of you *go outside* before you joined
the Army?" I knew a guy who thought that "seeing if the compass
points at it" is a good way to tell if the hill we're looking at is
the one we're supposed to find, fainted at the sight of his own blood,
and could barely carry the spare 240B ammo ten miles. (The 240B is a
medium machine gun.)

But no. No physics classes.

-Dan

[Phil]

On Mar 7, 7:35 pm, Simon Quellen Field <sfi...@scitoys.com> wrote:
> Did any of you take any physics classes in college?
> :-)

You apologized, then continued being snarky. A little smiley face
doesn't make insulting people okay.

But Simon is correct. The calculation is not carried out explicitly
in the Wikipedia article, but if you solve for velocity of the rifle
using conservation of momentum, and plug that velocity back into the
kinetic energy equation, you find that about 99.6% of the energy is in
the bullet.

[Simon Quellen Field]

Short emails can be interpreted in a variety of ways.

That is one of the problems with email, and why flame wars happen.

But you continue to interpret my comments as ill-intentioned, despite

my explaining that when I smile while asking a rhetorical question, it

is a friendly communication.

I was quite astonished to find that people can get degrees in biology

these days without having passed a basic physics course. When I made

the comment, I was expecting that the people I was talking to would then

pull out their old college physics book and remember what they had been

taught about basic kinematics. Or at least smile back.

To find that many of these quite intelligent people have never had a physics

course in either high school or college, and are making very basic mistakes

in simple physics, is quite surprising. But it makes it all the more important

to me to point out and correct bad physics when I see it in an arena such as this,

where people are going to learn about science. Someone will take it as truth

when it comes from a person who is obviously highly educated in biology, and

never know that this person was ignorant of basic physics.

When a bullet is fired, half the energy does not go into the gun.

Newton's third law is about force, not energy.

When you push against a wall, it pushes back with equal force, or else something

will move. Only when something moves has work been done, or energy transferred.

Underpinning all of biology is physics, and when building things like centrifuges,

it is dangerous to be ignorant of physics. I recommend that everyone on the list

who is still in college should take a basic physics course. And for anyone on the list

who has not taken physics, and is not still in college, a course at a local community

college is cheap, and will make you a better biologist, and a much better DIYbio

equipment builder.

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[Nathan McCorkle]

On Sat, Mar 10, 2012 at 6:23 PM, Simon Quellen Field <sfi...@scitoys.com> wrote:
> When a bullet is fired, half the energy does not go into the gun.
> Newton's third law is about force, not energy.
> When you push against a wall, it pushes back with equal force, or else
> something
> will move. Only when something moves has work been done, or energy
> transferred.

:)

Why do I sweat when I push on a wall then? Isn't that an effect
(side-effect?) of
energy transfer?

[Ethan]

It is an effect of energy transfer, of a sort, but the energy is not being transferred from you to the wall. Your sweating is a result of internal temperature increasing beyond the set-point of your hypothalamus. The heat is generated as a side product of molecular conversion of energy from chemical (ATP) to mechanical.The dynamic contraction and relaxation of individual myosin heads is what produces the force in an isometric muscle contraction. Macroscopically, the length of the muscle does not contract, but on the level of individual myosin molecules, they constantly are moving back and forth.

[Simon Quellen Field]

It isn't being transferred to the wall.

You are just heating up your environment.

But a better example is the brick exerting a force on the sidewalk.

Neither one sweats.

But the brick is exerting a force on the sidewalk, and the sidewalk is

exerting an equal and opposite force on the brick.

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[Nathan McCorkle]

On Sat, Mar 10, 2012 at 9:11 PM, Simon Quellen Field <sfi...@scitoys.com> wrote:
> It isn't being transferred to the wall.
> You are just heating up your environment.
>
> But a better example is the brick exerting a force on the sidewalk.
> Neither one sweats.
> But the brick is exerting a force on the sidewalk, and the sidewalk is
> exerting an equal and opposite force on the brick.

So gravity (constant force) vs no gravity doesn't effect the longevity
of materials? I.e. two equivalent steel beams, one stored in vacuum on
earth vs one stored in sub-gravity orbit for years and years....
assuming no oxidation, ionization, and only atomic decay.... the force
difference over the items' lives wouldn't have any molecular effect
(and I would assume that would imply energy transfer) ?