Re: car acceleration

["John S. Denker" <jsd@MONMOUTH.COM>]

Robert Cohen wrote:
> If a ball is thrown against a horizontal spring, the spring compresses and
> then extends.  The ball is then sent back in the other direction.  As the
> spring constant k goes to infinity, the spring appears to act like a wall.

OK.

> Does it?

Yes.

> Does the physics change?

Yes.

> Is energy stored in the k=infty spring
> when it is compressed, even though the amount of compression is
> infinitesimal?

No.  Not any significant energy.

==================

Specifically:  The physics changes when the compliance of the
wall becomes small compared to the compliance of the ball.

Or in the case of a skater:  The force on the wall is the
same as the force on the hand (disregarding minus signs)
which is the same as the force on the shoulder (disregarding
small terms such as the ratio of arm mass to torso mass).

So tell me, how far does the wall move when the skater pushes
with a force F?  And how far does the shoulder move?

I figure the ratio is at least 1000-to-one, closer to
1,000,000-to-one for really hard walls.  This is why
I think it is preposterous to focus on the energy stored
in the wall ( to the exclusion of processes internal to the
skater).