Car Rear Ends Truck
This animation depicts an inelastic collision between a truck and a car. In the collision between the truck and the car, total system momentum is conserved. Before the collision, the momentum of the car is 20000 kg*m/s and the momentum of the truck is 0 kg*m/s. The total system momentum is 20000 kg*m/s. After the collision, the momentum of the car is 5000 kg*m/s and the momentum of the truck is 15000 kg*m/s. The total system momentum is 20000 kg*m/s. Consistent with the law of conservation of momentum, the total system momentum is conserved. The momentum lost by the car (15000 kg*m/s) is gained by the truck.
An analysis of the kinetic energy of the two vehicles reveals that the total system kinetic energy before the collision is 200000 Joules (200000 J for the car plus 0 J for the truck). After the collision, the total system kinetic energy is 50000 Joules (12500 J for the car and 37500 J for the truck). The total kinetic energy before the collision is not equal to the total kinetic energy after the collision. A portion of the kinetic energy is converted to other forms of energy such as sound energy and thermal energy. A collision in which total system kinetic energy is not conserved is known as an inelastic collision.
For more information on momentum concepts and principles, visit The Physics Classroom Tutorial. Detailed information is available there on the following topics:
Momentum Conservation Principle
Momentum Conservation in Collisions