Throw the ball to yourself

[William Beaty <billb@eskimo.com>]

On Wed, 18 Feb 1998, Herb Schulz wrote:

> Howdy,
>
> The force the surface applies on your back is NOT a centripital force.  It
> is simply the contact force (Normal force) and there is a standard
> "reaction" force ON the surface (i.e.,. not on you) of equal magnitude and
> opposite direction.

True.  How about "basketball bouncing in a rotating cylindrical space
station" problem.  As a student this illuminated the "centrifugal force" 
fallacy for me.  Q: What force makes the basketball bounce?  A: None, in
an inertial reference frame the ball travels in a straight line between
collisions with the rotating cylindrical "floor."  Q: What force makes a
non-bouncing ball rest against the "floor"?  A: the floor is not at rest,
it is actually pushing on the ball to keep it going in a circular path,
and of course the ball pushes against the floor in return.  We can even
get rid of the circular path: give the ball a kick so it moves backwards
along the floor at the same speed that the floor moves forwards, and the
ball will become weightless.  From the rotating frame, it will appear to
skim across the cylindrical floor in a closed-loop path, without ever
touching the surface.

I vaguely recall my childhood concept about "centrifugal force".  I really
should have called it CENTRIFUGAL REPULSION FIELD.  A field after all is
not a force.  I thought that the rotation of the merry-go-round was
creating a sort of gravitational vortex field, and this field then pushed
my body outwards.  The forces I experienced were real, but the "field" was
not.  While in contact with the merry-go-round, my body pushed outwards
and the metal bars pushed me inwards.  Simple f=ma force (but only simple
if we can find a simple explanation of "a" besides the one which refers to
an absolute frame.)

Kids are right when they say that they feel a force throwing them
outwards.  They are wrong when they imagine that some sort of invisible
gravity-like field has been created by the rotating object.


I always had it in the back of my mind to write a "Pong" videogame which
would be played in a rotating environment.  Split the screen and show two
copies of the playing field, one from the rotating frame and one from an
inertial frame.  Let two players have at it.  Would it puncture their
"centrifugal force-field" misconception?  I think so, but knowing the
tenacity with which people hold on against conceptual change, I would have
to experiment to find out.


The Pacific Science Center in Seattle currently has one of those "physics
merry-go-rounds" which seats 8 people and is provided with a beachball.  I
found that if I threw the ball upwards but also inwards and backwards
against the direction of rotation, the ball would seem to bounce off of an
invisible column running down the axis of the chamber.  I could ALMOST
catch the ball as it returned (much to the consternation of the kids
seated across from me.)  In an inertial frame I was throwing the ball
across the diameter of the ride, while journying to the far position where
I could catch it.  With more practice I think I could have succeeded.  A
floppy beach ball and a high, horizontally-slow trajectory are of course
required. 

((((((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))))
William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  www.eskimo.com/~billb
EE/programmer/sci-exhibits          science projects, tesla, weird science
Seattle, WA   206-781-3320          freenrg-L taoshum-L vortex-L webhead-L