Re: [Phys-l] Work done by Static Friction

[John Mallinckrodt <ajm@csupomona.edu>]

On Mar 15, 2007, at 7:53 AM, LaMontagne, Bob wrote:

> ... Brian makes a point that might be what the texts are alluding  
> to. He
> shifted the question to what the stationary road is doing to the  
> tires.
> Here we have a static frictional force (with no displacement of  
> road or
> bottom of the tire) being applied -  and an associated acceleration of
> the truck. No one would argue in this case that the road was doing  
> work
> that resulted in the KE of the truck - the engine is ultimately the
> source of the motion.

I would.  I'm not saying there isn't room in my philosophy for this  
point of view.  One *can* calculate what some call the "center of  
mass work" done by the road and properly set it equal to the change  
in bulk kinetic energy of the truck.  But I find it somewhat  
distasteful to say that the KE of the truck was the "result" of work  
done by the road.

Better, in my view, to focus on the fact that the road provides a  
constraint and, as a result, an impulse that directly changes the  
momentum of the truck.  Unlike the center of mass work and the  
associated change in kinetic energy, the impulse provide by the road  
and the (equal) change in momentum of the truck are independent of  
the choice of reference frame and, as a result, more justifiably  
viewed as physically connected.

John Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://outlawsofphysics.com>