For a movie stunt, an empty truck with a mass of 2000 kilograms goes 10 meters per second and runs into a stopped car of mass 1000 kilograms The truck then keeps moving and pushes the car along with it. If there are no other forces acting on this system, which best describes the results of the collision?

With no external forces, momentum is conserved. The initial momentum is (2000 kg)(10 m/s) = 20,000 kg·m/s; after the collision, the 3,000 kg of vehicles move together at 20,000 ÷ 3,000 ≈ 6.7 m/s. This is less than the truck’s initial 10 m/s, so choice A is correct. Because the vehicles move together, the collision is not perfectly elastic, and kinetic energy does not stay the same.

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