Re: [Phys-l] Accelerating automobile

[Robert Carlson <rcarlson@physicstoolkit.com>]

This might be helpful, but only after energy and
rotation are covered.

http://physicstoolkit.com/mctc/lc00250.htm

Bob Carlson

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

> Thanks for the responses. I should have been more
> clear about where this question appears in the
> Serway text. It is posed immediately after
> introducing Newton's Laws but before the
> introduction of energy and momentum. The statement
> is that the frictional force between the tires and
> road "propels" the automobile (in the f=ma sense). I
> would think that a student would have a hard time
> justifying that for what is essentially a static
> friction force.
>  
> Bob at PC
>
> ________________________________
>
> From: phys-l-bounces@carnot.physics.buffalo.edu on
> behalf of John Barrer
> Sent: Sun 1/6/2008 10:21 AM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] Accelerating automobile
>
>
>
> 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
> > > _______________________________________________
> > Forum for Physics Educators
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> >
>
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