Re: Car acceleration

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Tue, 5 Feb 2002, Robert Carlson wrote:

> ajmallinckro@CSUPOMONA.EDU writes:
>
> > In this particular case I wouldn't do that because (ignoring air
> > resistance and rolling resistance) there *is* no force that is
> > doing "real work" on the car.
>
> Then you would say there is no real work being done?

Right.

> And no real change in kinetic energy?

No; obviously the kinetic energy is changing.

Let me say it all again right here in one place:

1. The static frictional force does "pseudowork" on the car.

(where "pseudowork" = external force dotted into the displacement
                      of the center of mass of system)

2. The pseudowork is equal to the change in the bulk translational
kinetic energy of the car.

(This is because static friction is the ONLY force that does
pseudowork on the car--neglecting any drag forces--and because net
pseudowork is ALWAYS equal to the change in the bulk translational
kinetic energy of a system.)

3. The static friction does no "real work" on the car

(where "real work" = external force dotted into the displacement
                     of the point of contact.

4. Thus, the total energy of the car does not change.

(Because NO "real work" is done on the car and the net real work
is ALWAYS equal to the change in *total* energy of a system as
long as there is no heat transfer to or from the system.  This is,
however, a bit of a stretch in this case and, beyond neglecting
the drag forces, also requires us to pretend that the oxygen used
in the combustion processes as well as the exhaust products remain
part of the system.  One could imagine a car that carries its own
oxygen tanks and diverts its exhaust into a big bag or something
like that!  This statement is also contingent on the assumption
that we take into account "thermal energy" and that there are no
thermal losses from the radiator or any other source.  The
pseudowork analysis is FAR simpler because it is subject to far
fewer caveats of this nature.)

> I would say there is a real force doing work and this comes
> from the drive axle applying a normal force on the car.

I'd need a far clearer picture of what your intended "system" is
and what "external agents" are responsible for the work you are
talking about before I could make sense of this statement.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm