http://www.ck12.org Chapter 9. Energy
9.4 Elastic and Inelastic Collisions
- Describe the difference between elastic and inelastic collisions and indicate what is conserved in each case.
- Solve problems involving elastic collisions using both the conservation of momentum and the conservation of
kinetic energy.
This device is known as Newton’s cradle. As the balls collide with each other, nearly all the momentum and kinetic
energy is conserved. If one ball swings down, exactly one ball will swing up; if three balls swing down, exactly
three will swing back up. The collisions between the balls are very nearly elastic.
Elastic and Inelastic Collisions
For all collisions in a closed system, momentum is conserved. In some collisions in a closed system, kinetic energy is
conserved. When both momentum and kinetic energy are conserved, the collision is called anelastic collision.Most
collisions are inelastic because some amount of kinetic energy is converted to potential energy, usually by raising
one of the objects higher (increasing gravitation PE) or by flexing the object. Any denting or other changing of
shape by one of the objects will also be accompanied by a loss of kinetic energy. The only commonly seen elastic
collisions are those between billiard balls or ball bearings, because these balls do not compress. And, of course,
collisions between molecules are elastic if no damage is done to the molecules.
Much more common areinelastic collisions. These collisions occur whenever kinetic energy is not conserved,
primarily when an object’s height is increased after the collision or when one of the objects is compressed.
Example Problem:A 12.0 kg toy train car moving at 2.40 m/s on a straight, level train track, collides head-on with
a second train car whose mass is 36.0 kg and was at rest on the track. If the collision is perfectly elastic and all
motion is frictionless, calculate the velocities of the two cars after the collision.