85.2 Newton's second law and third law of motion 2. Newton's third law of motion If a system A exerts a force on another system B, then B exerts a force of the same magnitude on a but in the opposite direction. Bon a a third law force pair are equal magnitude and opposite direction, but act on different systems 85.3 the limitation to applying Newtons law of motion 85.3 The limitation to applying Newton's law of motion 1. Reference frames Newtons law can only be used in inertial reference frames(that are not being accelerated 2. Speed limits Newtonian mechanics is a good approximation as long as the speed of the system is much less than the speed of light.4 2. Newton’s third law of motion If a system A exerts a force on another system B, then B exerts a force of the same magnitude on A but in the opposite direction. FA on B FB on A r r = − A third law force pair are equal magnitude and opposite direction , but act on different systems. §5.2 Newton’s second law and third law of motion §5.3 the limitation to applying Newton’s law of motion §5.3 The limitation to applying Newton’s law of motion 1. Reference frames Newton’s law can only be used in inertial reference frames (that are not being accelerated). 2. Speed limits Newtonian mechanics is a good approximation as long as the speed of the system is much less than the speed of light