## Truck Rear Ends Car

### Elastic Collision

A GIF Animation

Collisions between objects are governed by laws of momentum and
energy. When a collision occurs in an isolated system, the total
momentum of the system of objects is conserved. Provided that there
are no net external forces acting upon the objects, the momentum of
all objects before the collision equals the momentum of all objects
after the collision. If there are only two objects involved in the
collision, then the momentum lost by one object equals the momentum
gained by the other object.

Certain collisions are referred to as elastic collisions. Elastic
collisions are collisions in which both momentum and kinetic energy
are conserved. The total system kinetic energy before the collision
equals the total system kinetic energy after the collision. If total
kinetic energy is not conserved, then the collision is referred to as
an inelastic collision.

The animation below portrays the elastic collision between a
3000-kg truck and a 1000-kg car. The before- and after-collision
velocities and momentum are shown in the data tables.

In the collision between the truck and the car, total system
momentum is conserved. Before the collision, the momentum of the
truck is 60000 kg*m/s and the momentum of the car is 0 kg*m/s; the
total system momentum is 60000 kg*m/s. After the collision, the
momentum of the truck is 30000 kg*m/s and the momentum of the car is
30000 kg*m/s; the total system momentum is 60000 kg*m/s. The total
system momentum is conserved. The momentum lost by the truck (30000
kg*m/s) is gained by the car.

An analysis of the kinetic energy of the two objects reveals that
the total system kinetic energy before the collision is 600000 Joules
(600000 J for the truck plus 0 J for the car). After the collision,
the total system kinetic energy is 600000 Joules (150000 J for the
truck and 450000 J for the car). The total kinetic energy before the
collision is equal to the total kinetic energy after the collision. A
collision in which total system kinetic energy is conserved is known
as an elastic collision.

For more information on physical descriptions of motion, visit
The Physics Classroom. Detailed
information is available there on the following topics:

Momentum
Momentum Conservation
Principle

Isolated
Systems

Momentum Conservation
in Collisions

Kinetic Energy

Other animations can be seen at the Multimedia
Physics Studios.

© Tom Henderson, 1996-2007

All Rights Reserved