Question

An automobile of mass 1500kg moving at 25.0m/s collides with a truck of the mass 4500kg at rest. The car and the truck bounce off each other completely elastically. Compare the force exerted by the car on the truck with that exerted by the truck on the car during the collision.

Answer 1

Answer: Both forces are equal each other.

Explanation:

If we assume that no external forces act during the collision (based on the fact that the collision time is very small), total momentum must be conserved, and Newton's 3rd law must apply.

If the total momentum is conserved, the change in momentum for the automobile (caused by the truck) must be equal to the one caused to the truck by the car.

This change in momentum is caused by internal forces, which must be equal each other (even though of opposite signs).

Another way to explain this, is that by Newton's 3rd Law, the force applied by the automobile to the car, must be equal and opposite to the one the truck is applying to the car at the same time.