text: why is it believed that force is the cause of motion,how does that compare to einstein's law of motion?   f=ma 1st thing is it einstein's law of motion or new ton's any way:  we kw that a=v/t^2  and e=mc^2  c is velocity~~ square of speed of light  nw compare the two!  without any source of force there is absolutely no motion! but a substance suppressing or stoping motion occurs when an equal and opposite force is applied!