A 12 kg box is pulled to the right by a 350 N force that is 30 degrees above the horizontal. A constant frictional force of 120 N opposes it. What is the net horizontal force acting on the box?

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 •