Momentum and Collisions

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 20 000 kg*m/s and the momentum of the truck is 0 kg*m/s; the total system momentum is 20 000 kg*m/s. After the collision, the momentum of the car is 5 000 kg*m/s and the momentum of the truck is 15 000 kg*m/s; the total system momentum is 20 000 kg*m/s. In accord with the law of conservation of momentum, the total system momentum is conserved. The momentum lost by the car (15 000 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 200 000 Joules (200 000 J for the car plus 0 J for the truck). After the collision, the total system kinetic energy is 50 000 Joules (12 500 J for the car and 37 500 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.

The animation was created on a Macintosh using the Interactive Physics program from Knowledge Revolution. The animation was then saved as a QuickTime movie. The above image is merely a screenshot of the first frame of the QuickTime movie. The actual QuickTime movie can be viewed within your Web browser using the appropriate plug-in or viewed in a separate application window using a helper application; the buttons below leave you with the choice as to how you would like to view the movie.

Information is available (at other sites) about the following topics:

For more information on physical descriptions of ray optics, visit The Physics Classroom Tutorial. Specific information is available there on the following topics:

• Momentum
• Momentum Conservation Principle
• Isolated Systems
• Momentum Conservation in Collisions
• Kinetic Energy