Re: [Phys-l] Accelerating automobile

[John Barrer <forcejb@yahoo.com>]

I would suggest this is a case where energy analysis
really helps to complete the picture. While the road
provides the net force which accelerates the car due
to an N3 interaction, it is the internal energy
(chemical) of the car which is the source of the
increase of the car's KE. It is this internal energy
which enables the car to exert a push on the road.
Energy pie charts of the car/road/Earth/air system
(other systems can be chosen as well but this choice
keeps the size of the pie constant) for two states
illustrate what's happening. State 1 - car at rest.
Entire pie is chemical energy. State 2 - car now
traveling at steady speed. There is now a small slice
of KE, a larger (appx 3X) slice of dissipated energy
(call it something else if this term offends), with
the balance of the pie chemical. Sizes of the slices
need to be qualitatively appropriate. A discussion of
where this dissipated energy "resides" can be quite
rich. 

John Barrere
Fresno USD

--- "LaMontagne, Bob" <RLAMONT@providence.edu> wrote:

> My department just switched to the latest version of
> Serway - Physics for Scientists and Engineers. The
> following problem is presented in chapter 5 (prob
> 26):
>  
> "A car is moving forward slowly and is speeding up.
> A student claims "the car exerts a force on itslf"
> or "the car's engine exerts a force on the car"
> Argue that this idea cannot be accurate and that
> friction exerted by the road is the propulsive force
> on the car."
>  
> How would members of this list respond to this
> problem? If I am standing on ice and push off from a
> rigid wall, is it the wall providing the propulsive
> force on my torso or is it the muscles in my arm?
>  
> Bob at PC
> > _______________________________________________
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>