Re: [Phys-l] check your work (and kinetic energy)

[Carl Mungan <mungan@usna.edu>]

> Date: Sat, 07 Jan 2012 23:14:07 -0600
Brian W wrote:

> I'm probably missing something when I say:  when the car moves right,
> the Earth necessarily moves left.     Later, with the brakes applied,
> the car slows to a halt, the Earth moves to the right, and warm air from
> the disks wafts into the breeze.

Depends what you mean by "miss something" -- maybe you should draw your statements to a conclusion so we can decide if you're missing anything. What you've said *so far* is certainly fine.

Yes, there are equal and opposite forces on car and earth. Ditto for momentum changes.

But you weren't going to go on and say the car and earth have equal changes in kinetic energy, were you? Even from a pseudowork viewpoint, although the magnitudes of the forces on car and earth are equal, the displacements of their center of mass are way different, right?

As for the warm air, that refers to overall energy conservation and has little to do with notions of work and mechanics principles.

I presume you're good with all this, but maybe someone else isn't, which is why I'm trying to complete the flow of thoughts you started. -Carl