# Feynman says "The total work done in going around a complete cycle should be zero for gravity forces"

I am reading "The Feynman lectures on physics volume 1".

Feynman says

"The total work done in going around a complete cycle should be zero for gravity forces"

When gravity forces are given, I think we cannot move an object around an arbitrary complete cycle.

I cannot understand what Feynman is saying.

• Why can't you? You can apply external force to do that.
– user243267
Dec 21, 2019 at 13:04
• What about Earth moving around the Sun? Dec 21, 2019 at 13:31
• By the way Feynman's statement should be true only for static fields. For time dependent gravitomagnetic fields there should be an anologon of induction. Dec 21, 2019 at 13:31
• What he is saying is simply that for conservative forces the work around a loop is zero. Because such forces can be written as the gradient of a potential and $dW$ is an exact differential (assuming space is simply connected). The converse is even more true: if the work around any loop is zero the force field is conservative.
– lcv
Dec 21, 2019 at 13:48