Sometimes as I am reading about the history of physics I run across this phrase. Ponderomotive force. I have tried looking it up several times but can never find an answer that explains what is meant by this phrase. Can someone tell me in laymens terms what is a Ponderomotive Force? Sometimes people like Einstein, Lorentz and Maxwell use this phrase. Give me an example of what this force is and how it works. I am looking for the mechanical details of what it is. Thanks for your help.