## Car Rear Ends Truck

### Inelastic 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 inelastic
collision between a 1000-kg car and a 3000-kg truck. 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 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. 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 objects 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 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