a law concerning the speed at which planets travel; a line connecting a planet to the sun will sweep out equal areas in equal times "Kepler's second law means that a planet's orbital speed changes with its distance from the sun"
(mechanics) Newton's observation that the rate of change of the momentum of a body is directly proportional to, and in the same direction as, the net force acting on it. Often stated as F = ma: F = force, m = mass, a = acceleration.