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Re: Car acceleration



1) Scott Goelzer wrote that according to some students,
it is the _engine_ that makes the car go.

The students may be 100% correct, depending on
details of what the question was.

2) Many others have pointed out that it is the friction
between the tire and the road that makes the car go.

That is also a 100% correct answer ... to a
different question.

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

The two questions are, respectively:

1) Where does the energy come from?

2) Where does the momentum come from?

THESE ARE TWO DIFFERENT QUESTIONS WITH TWO DIFFERENT ANSWERS.


The momentum must flow in from outside the car,
because you can _never_ store or hide momentum.
There is no on-board "tank of momentum". In
contrast, there is an on-board tank full of
energy.

The energy must come from an internal source,
because the tire/road contact is quasi-static
and (in the lab frame) cannot transfer any
energy. Work is F dot dx, where dx is zero. In
contrast, momentum is F * dt, where dt is
distinctly nonzero.

Gary Turner wrote:

In my last post, I said that static friction can do no work - that was
intended to be applied to this case only and I still maintain that.

We agree that static friction does no work in
this case. (Other cases may require additional
analysis.)

The
site of the frictional force does not move, the frictional force is not
applied over any distance, it does no work.

Righto.

Consider the cylinder (or wheel) rolling down a slope. There you must have
static friction, but it doesn't do any work. It can easily be shown that
the gain in K = loss in U(grav) when rotational kinetic energy is
included. Why is this any different? Just because there is an engine
applied?

Different? What is or isn't different from what?
-- The part about (quasi)static friction is not different.
-- The part about gravitational energy being converted to
KE is different, precisely because of the engine, which
converts fuel energy to KE.

The case of work only complicating things is somewhat valid. Certainly by
Newton's Laws and the extent of the FCI, the only force that can cause an
acceleration is the static friction. That is why, at first glance, answer
C was (the most) correct. Case closed.

Case closed, because the original question was
asking (with somewhat less than perfect clarity)
about the force, not the energy.

However, in our course, work comes
about 3 lectures after Newton's Laws and this is still very fresh in the
mind.

So that will tend to distract students and
trick them into misreading the question.

How can something start to move when there is no force capable of doing
work on it?

I suggest rephrasing that question in more technical
physics terms. Let's not say "start to move". Rather
let's ask
1) Where does the energy come from?
2) Where does the momentum come from?

These are two different questions with two different answers.

The energy has to come from somewhere - where? Ok, ultimately
it comes from the fuel, which came from the sun, which is a result of
matter annihilation - but that is irrelevant.

Agreed: irrelevant truisms.

There has to be a force that
is the intermediary to transfer the energy (via work) into kinetic energy.

Well, not quite. There has to be a _conversion_
of energy, not necessarily a _transfer_ of energy.

This is a perfect illustration of why I keep saying
things like
-- momentum is primary and fundamental
-- energy is primary and fundamental
-- work is not.

Work is secondary at best. You can talk about work
_if_ it helps you describe what the energy is doing.
OTOH if work does not describe what the energy is
doing, forget about work.

Remember that the work-KE theorem applies _only_ to
point particles (objects with no internal structure)
which certainly excludes car engines. So we have no
way to say anything about the work of the engine. Who
cares about work? We know everything we need to know
about the energy budget without calculating the work,
so forget about work!

Maybe this is not static friction after all.

It's essentially static. The slight non-static
effects are totally irrelevant to this problem.

At what point does the
enormous number of contact points moving zero distance (=zero work) change
to an enormous number of contact points moving an infinitesimal distance
(=finite work).

Well, even if they did do nonzero work, it would
be in the form of slippage, taking energy _out_ of
the system. I cannot conceive of a way that
nonidealities in the tires would help add energy
to the car. (Unless you've got a supply of flubber.)

How does a unicycle work?

1) The energy comes from inside the unicycle+rider system.
2) The momentum comes from outside the system.

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

Tangential remark:

Note that in this breakdown:
-- energy from inside
-- momentum from outside
the pseudowork behaves more like momentum transfer
than like real energy transfer. This leads me to
conjecture that pseudowork is mostly a complicated
roundabout way to talk about momentum transfer.

Momentum is primary and fundamental.