Re: Car acceleration

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Mon, 4 Feb 2002, Gary Turner wrote:

> From a FBD, it appears as though the friction between the car and the road
> provides the force to accelerate the car.  But what is that friction?  It
> is not kinetic because these surfaces are not sliding.

Right.

> It cannot be static, because static can do no work

This isn't true in general.  Consider the case of a refrigerator
in the bed of a pickup truck as the truck accelerates (slowly!)
after stopping at an intersection.  The refrigerator has genuine
"work" done on it by the static frictional force applied to it by
the bed of the pickup truck.

In the case under consideration, however, it is true that the
static frictional force from the road does no "work" (in, perhaps,
the most common sense of the word "work" and in the frame of the
road.)  Accordingly, the total energy of the car remains fixed
(ignoring for now other influences like air resistance.)  Some of
the energy of the car changes form from chemical potential in the
gas to bulk kinetic energy of linear motion.  The road merely
serves as a necessary constraint to mediate that internal change.

On the other hand the frictional force does do what some people
like to call "pseudowork" or "center of mass" work (i.e., F_net *
displacement of the CM) and this quantity can always be equated to
the change in the bulk linear kinetic energy.

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