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

On 1/8/2012 1:36 PM, Carl Mungan wrote:
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

Let me attempt to talk myself into a corner then.
I suppose that there is a large reservoir of energy available in the gas tank, and a small reservoir of energy available in the battery and a large energy sink in the radiator.
I suppose that the engine converts some chemical energy to dispersal at the radiator, and some to driving a transmission and wheels. I know quite certainly that in a RWD car, the back axle presses on its suspension, via cartwheel springs or wishbones of some kind.
This makes an effective lever between the instantaneous road service patch used for traction
and the gear teeth then in contact with the transmission shaft.
At steady speeds, this force is applied over a patch moving at some speed over the ground,
whose units represent power at the road surface.
With the throttle released, the wheels begin to drive the transmission, so that the engine winds down more slowly than it would in neutral. When the need to slow is more pressing, the brakes act to apply a retarding force on a road patch at some reducing speed, so that power is converted from kinetic energy to brake and tire heating.
It is easy for me to model a force, and a speed in either case, and I take any force applied over some distance to represent work, and its rate to represent power.

I can suppose that it is my too easy acceptance of a force moving over some distance that is the cause of contention?

Brian W