Re: positive and negative work

[Brian Whatcott <inet@INTELLISYS.NET>]

> x
> > >So the car exerts a force on itself?
> >
> > ... Yes, the car engine exerts a force which can resist this
> > tendency to accelerate down hill.
>
> I disagree.  It is the road that exerts that force.
/snip/
> John Mallinckrodt      mailto:ajm@csupomona.edu
> Cal Poly Pomona          http://www.csupomona.edu/~ajm


Hmmm... first I say thanks to John, for patiently rehearsing
what he has presumably said many times.

I think I see why people sound as querulous at the nth iteration
as they did at the first iteration of the work discussion.

John's view on forces is exactly analogous to the following scenario,
as far as I can concretely see.

I apply a nut and a bolt to a hole drilled in two sheets of metal, in order
 to hold them together.

For John it is (apparently) not the bolt's extension which supplies
a clamping force, rather, it is the contraction of the two sheets between
the nut and  bolt which provide the force.  I expect I missed something,
 because I find  that pointing to the bolt as the first cause of the force
 is at least as tenable as the squeezed material.

In the case in question, I point to the car's mass as providing a downhill
force.
If I connect the car's engine to an air propellor pointing uphill, then I
slow the
downhill gallop,  though there be no traction at the tires. This evidently
contradicts the idea of forces originating at a road contact. In this case,
they do not.




Brian Whatcott
  Altus OK                      Eureka!