Power P is the rate of change d W /d t of the work W, as the trajectory is extended by a position change d x in a time interval d t : d W = d W d x ⋅ ⋅ d x = F ⋅ ⋅ d x , so P = d W d t = d W d x ⋅ ⋅ d x d t = F ⋅ ⋅ v , with v = d x / d t the velocity.