Originally Posted By: Bill

Rather than saying force imparts energy a better way to say it is to say that force transfers energy.... from one part of the system to another.

I agree with what you say, but some clarification regarding the nature of the system is required. Earth is system on its own. Moon is a system on its own. However, the earth and the moon together form a system.

On the earth, transfer of energy is possible by collisions between atoms. But between earth and moon no such collision is possible. So when gravity acts, no energy transfer can take place between the two. But, it is held that gravity imparts energy to the moon. The orbit of moon is slightly elliptical, and the variation in speed is taken as a consequence of energy being imparted. That is, during a revolution, the total energy of the moon remains varying .

What I argue is that just because force exists between earth and the moon, there is no transfer of energy between the two. The slight variation in the speed of the moon is due to transfer of energy within the moon. That is, even though the speed varies, the energy of the moon always remains the same. I think you have understood my point.

The above difference, I think, is due to the mathematical definition that F= ma, which implies that force acting on a distance imparts energy.