Truck Rear Ends Car
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 truck is 60 000 kg*m/s and the momentum of the car is 0 kg*m/s. The total system momentum is 60 000 kg*m/s. After the collision, the momentum of the truck is 45 000 kg*m/s and the momentum of the car is 15 000 kg*m/s. The total system momentum is 60 000 kg*m/s. Consistent with the law of momentum conservation, the total system momentum is conserved. The momentum lost by the truck (15 000 kg*m/s) is gained by the car.
An analysis of the kinetic energy of the two vehicles reveals that the total system kinetic energy before the collision is 600 000 Joules (600 000 J for the truck plus 0 J for the car). After the collision, the total system kinetic energy is 450 000 Joules (337 500 J for the truck and 112 500 J for the car). 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
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