A railroad car with a mass of 63,000 kg coasts along a level track at 1.3 m/s. It hits a stationary car with a mass of 50,000 kg. The cars connect together when they collide. How fast are they moving after the collision? A teenage driver backs into a bush at 2 mph. The impact stops the car. The car is not damaged. Later that day, they back into a fencepost at 2 mph. The impact stops the car. The bumper is dented by the fencepost. Use physics terms to explain why the bumper is not dented when the car backs into the bush.