Re: Car acceleration

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Tue, 5 Feb 2002, John S. Denker wrote:

> But it would be so much easier to describe the
> center-of-mass motion of the car in terms of momentum.
> And there is no need to mention thermodynamics in
> connection with this problem.

Let's compare on this very simple problem.

A car of mass m starts from rest and accelerates due to a constant
static frictional force F on the tires.  How fast is it going
after moving a distance d?

1. Using pseudowork = change in KE

      Fd = mv^2/2 => v = sqrt(2Fd/m)

2. Using impulse = change in momentum

      Ft = mv     => v = Ft/m

   But we aren't given t so we'll have to do some more stuff.

Granted, this isn't rocket science, but as Carl said, why deny
ourselves the use of pseudowork if it solves even one problem in
such a straightforward manner, especially when most texts would
work this problem exactly as I have and claim that they are using
*the* work-energy principle!

> BTW, as I reported previously, I've asked a number
> of real live physics researchers if they could
> define "pseudowork" for me.  Precisely none of them
> had ever heard the term (let alone used it).

And as you have also correctly pointed out on many occasions the
private wishes, desires, practices and biases of individuals, no
matter their credentials, are utterly irrelevant.  Only the
verdict of nature matters and within the realm of Newtonian
mechanics nature *always* obeys the relationship between
pseudowork and change in bulk translational kinetic energy.

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