A 2.0-kg stone is tied to a 0.50-m long string and swung around a circle at a constant angular velocity of 12 rad/s. The net torque on the stone about the center of the circle is:

Consider a system to be two train cars traveling toward each other. what is the total momentum of the system before the train cars collide?   kg • what must the total momentum of the system be after the train cars collide?   kg •