Fun in Space
Donald E. Simanek
article by Lee DuBridge. In the early 60s I often gave my students,
especially those who intended to be teachers, documents to 'clean up'.
Some I'd make up myself, some were found in newspapers, some in magazines,
and some in journals. The students were to act as technical proofreaders,
correcting physics errors, tightening up the language, clarifying the
explanations.
But when the article was by a famous and renowned personage, as this one
by DuBridge, some students hesitated to correct the 'great man'. But others
took delight that such a person could be so careless. Some of the better
students wondered how he 'got away' with such a 'lightweight' talk to an
audience of physicists. I told them that he probably wasn't paid much
for it.
A psychologist of the 50s (Ann Roe, I think) did a study of people in
various professions, and their personality and other traits. She noted
that physicists were notoriously imprecise and careless when speaking with
each other, leaving out crucial steps, using words imprecisely, etc. This
contrasts with the popular image of the 'precise' scientist. She
thought this was because, when speaking with those who shared the same
background, they could count on the listeners immediately correcting the
errors and filling in the gaps, without comment, without even being
conscious that they were doing it. They were also impatient with what
they considered 'trivial' details which 'everyone knew'.
Unfortunately, many of them do the same when teaching undergraduates.
This, I think, accounts for why physics is one of the worst-taught
subjects in colleges. But many profs defend this, saying that it helps
weed out those students who can't catch on by themselves.
[Dr. Roe also observed a difference between mathematicians and physicists.
When physicists reached the point where they became convinced that there
*was* an answer to a problem, they lost interest in it and turned it over
to a graduate student. When mathematicians tackled problems they saw them
through to the bitter end.]
The following is just one small excerpt from a much longer speech. The
example is good, and the technical description is essentially correct.
It degenerates in the last two paragraphs, however.
FUN IN SPACE*
Lee A. DuBridge
California Institute of Technology Pasadena, California
(Received May 7, 1960)
This after-dinner address attempts to point up in a simplified way
the amusing, as well as some of the more serious, problems which
arise in connection with flight into space. Figures are given to
challenge some of the more fanciful claims about the value of the
moon as a military base, noting the very large amounts of fuel
required to take weapons up to the moon and then to return them to
the earth. Some of the important research problems in physics and
astronomy which can be carried out by the use of space vehicles are
enumerated As examples of a apace science enterprise, the results
of the magnetic field measurements by the Pioneer V package are
summarized The impracticability of using the moon and other planets
as colonies for the earth's excess population is also demonstrated.
___________________________________
A wonderful thing has happened during the past three years A new
subject has been opened up which even an old-fashioned physicist
can understand, A new subject that involves no relativity
corrections. No strange-particle theory--not even any Fermi
statistics. Just good old-fashioned Newtonian mechanic s ! Space
! All you have to do is get an object a couple of hundred miles
above the earth and give it a horizontal speed of 5 or 10
miles/sec, and from that time on you can tell exactly what's going
to happen to it--maybe even for a billion years--by just using
Newton's laws of motion and his law of gravitation. The
mathematical details get a little rough now and then, but a good
IBM machine will take care of that--if you can find someone who
knows how to use it. But there is nothing in principle that any
physicist can't understand, I personally prefer to talk about
space to non-scientific audiences In the first place, they can't
check up on whether what you are saying is right or not. And, in
the second place, they can't make head or tail out of what you are
telling them anyway--so they just gasp with surprise and
wonderment, and give you a big hand for being smart enough to say
such incomprehensible things. And I never let on that all you have
to do to work the whole thing out is to let the centripetal force
equal to the. gravitational force and solve for the velocity.
That's all there is to it! Knowing v, you can find the period of
motion, of course, and, that's practically all you need. To show
what I mean, let me give a simple example that I heard discussed
at an IRE meeting a couple of years ago,
Imagine two spacecraft buzzing along in the same circular orbit
around the earth--say 400 miles up--and one ship is 100 yards or
so ahead of the other one. The fellow in the rear vehicle wants to
throw a baseball or a monkey wrench or a ham sandwich, or
something, to the fellow ahead of him. How does he do it?
It sounds real easy. Since the two ships are in the same orbit.
they must be going at the same speed--so the man in the rear could
give the baseball a good throw forward and the fellow ahead should
catch it.
But wait! When you throw the ball out, its speed is added to the
speed of the vehicle so not only is it going too fast for that
orbit; and the ball goes off on a tangent and rises to a higher
orbit. But an object in a higher orbit must go slower. in fact, the
faster he throws the ball, the higher it rises and the slower it
goes. So our baseball pitcher stares in bewilderment as the ball
rises ahead of him, then seems to stop, go back over his head, and
recede slowly but surely to the rear, captured forever in a higher
and slower and more elliptical orbit while the pitcher sails on
his original course.
You must make a correction, of course, if you assume the ball's
mass is not negligible and you take account of the conservation of
momenta. Then, as the ball is pitched forward, the vehicle is
slowed dawn--whereupon it falls into a lower orbit where, of
course, it goes faster So in this case the ball appears to rise
higher and fall behind faster.
But now our ball thrower decides to try again. This time he is
going to be smart. If you can't reach the guy ahead by throwing
forward, the obvious thing to do is throw the ball to the rear. Now
its speed is subtracted from that of the vehicle; hence it is going
too slow for its orbit; hence it falls to a lower orbit and goes
faster, passing underneath the rear vehicle, moves forward and
passes underneath the forward vehicle, and then on into its orbit.
It will be left as an exercise for the student to determine just
how the baseball may be launched in order to hit the forward
vehicle. One way, of course is to first circle the earth and come
back on the second lap, but there are other ways.
Now, that's all very simple Newtonian mechanics, of course. But
you can see how, when you start to explain that to make an object
go faster you slow it down and to make it go slower you speed it
up, people begin to think you are either crazy or very smart.
However, tonight I am talking to physicists and they are used to
far crazier things than that--so they will have no trouble
believing me at all.
There is another bundle of space problems that can be a source of
considerable amusement. Have you ever tried to explain to your wife
why it as that if she were in a space capsule in an orbit around
the earth she would have lost all her weight? Now the idea of
losing a few pounds of weight might appeal to her but I am sure the
notion of weightlessness is something incomprehensible to most
people. If you ask moat laymen why the condition of weightlessness
exists they would tell you that since you are above the earth's
atmosphere there isn't any gravity and so, of course, you must be
weightless. To such people one must carefully explain that the
force of gravity 200 miles above the surface of the earth is only
10% less than it is on the earth's surface. Even at 4000 miles the
gravity is reduced only to one quarter of its value on the earth
surface; and at 8000 miles, to one-ninth. Since it is obviously
gravity that holds a satellite in a circular orbit, and since the
earth's gravity is even strong enough at the distance of the moon-
-240,000 miles--to hold the moon in its orbit, the weightlessness
in an earth satellite is evidently not caused by the absence of
gravity.
Then what is it caused by? Of course, if you want to be a real
coward you will choose the easy way out and simply say that in a
circular orbit the force of gravity is canceled by the centrifugal
force, and the condition of weightlessness results. You know very
well of course, that that isn't the proper explanation. The
centrifugal force is the force that the satellite exerts on the
earth and is not a force on the satellite. The force on the
satellite is toward the earth and, indeed it is the force of
gravity which supplies the centripetal force which keeps the
satellite in its orbit. In other words, gravity and centripetal
force are in the same direction, not opposite. So when this is
pointed out by some unkind person you get more sophisticated and
say simply, "Well, in any freely faIling object the condition of
weightlessness exists. It would exist, for example, for passengers
in a freely falling elevator." But since not many people have been
passengers in a freely falling elevator, this explanation usually
falls fairly flat also. At this point I recommend that the argument
be abandoned and we retreat into technical jargon by saying, "Well,
it's just one of Newton's laws of motion that whenever the inertial
reaction and the accelerating force are equal no tendency toward
further acceleration can exist, and hence the system behaves as
though no gravitational field were present." No one can quarrel
with that statement. Even if nobody understands it, it's true.
* Text of remarks at the Banquet of the l960 Spring Meeting of the
American Physical Society, Sheraton Hall, Washington, D.C., April
27, 1960.
[End of DuBridge excerpt.]
-- Donald