Re: car acceleration

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

John Barrer wrote [partly off list]
...
> Don't you transfer energy into the wall
> when you push it (thereby deforming it), and then that
> energy is recovered as KE as the wall pushes on you?

No.

This question has come up several times.  The
answer is still no.

There seems to be a widespread preconception that
the wall "must" transfer energy to the skater
who pushes on the wall.
This is not what typically happens.
This is not even approximately what happens,
unless you have incredibly hard hands and/or
incredibly soft walls.

I say again, under realistic conditions:

 -- The transfer of momentum from wall to skater
is large.

 -- In contrast, the transfer of energy from
wall to skater is zero on any practical scale,
and slightly negative if you look closely enough.
The magnitude of the temporary transfers is
small, and the magnitude of the net transfer is
smaller still.

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

> The issues of where the KE "came from" and how exactly
> that net external force arose are more subtle and will
> be very difficult for most HS students to comprehend.

What's subtle?
What's difficult?

The momentum came from an external souce (road via tires).
The energy came from an internal source (fuel via engine).
The PE to KE conversion was internal to the car.

A large momentum transfer need not be associated
with a large energy transfer.  E = p^2/(2m).

High-school students should be able to handle p^2/(2m).

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

This may be a symptom of a more fundamental
misconception.  In general, people seem to be
hypnotized by energy _transfers_ to the
neglect of energy itself.

Yes, you can contrive an atypical scenario
where energy is _transferred_ to the trampoline
and _transferred_ back.  But that is a
bizarre contrivance.  It has neither the
advantage of simplicity nor the advantage
of accuracy.  What really happens is an
_internal_ conversion of PE to KE, internal
to the skater.

Fixating on energy _transfers_ is a problem.
It's a nuisance in basic mechanics, and it's
a disaster in thermodynamics.