[Jason_b]

I wish I found this thread earlier, but I was out to dinner.

Car wheels against the road: all that is happening is mechanical energy is being released in a way that moves the car forward. Wheel to road is just another example of clutch to flywheel or gear to gear, or gear to driveshaft.
The energy comes from chemical potential energy in the liquid hydrocarbon fuel being combusted and converted into heat and gas which pushes the piston.

With your foot and the ground, that is also merely transmission, the energy comes from metabolism and the processing of sugars.

The key point was said in an earlier post referencing Newton's law, "III. For every action there is an equal and opposite reaction." If you were floating free in space and had a bowling ball in your hand, and you threw it away with vigor, the ball would be set in straight line motion away from you, but you would be moving away from the point of release as well, in a straight line and in the opposite direction. You will have propelled yourself. And you would continue on in that line and the ball opposite forever until some force acted to stop you or change your directions.

Now take an amount of gas the total mass of which is equal to the ball, compress it, and release it suddenly while floating in space. You acceleration however brief, a better word would be impulse, would be the same as if you had thrown the ball. A rocket engine produces thrust that is as if you had a limitless supply of those impulses (until the fuel runs out) laid them end to end so there is no beginning or end, just a stream.

[cinFLA]

You acceleration however brief, a better word would be impulse, would be the same as if you had thrown the ball.

Only if the total momemtum of each is equal. Your acceleration would depend in one case on the amount of pressure and in the other on how hard you threw the ball.