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 also energy is conserved. When both momentum and energy are conserved, the collision is called an elastic collision. Most collisions are inelastic because some amount of energy is converted into another form. Any denting or other changing of shape by one of the objects will also be accompanied by a loss of 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 are inelastic collisions. These collisions occur whenever energy is not conserved, primarily when an object's height is increased after the collision or when one of the objects is compressed.
The following video is a demonstration of elastic and inelastic collisions. Use this resource to answer the questions that follow.
- Explain what happened in the first demonstration on elastic collisions.
- Explain what happened in the second demonstration on inelastic collisions.
- Assuming the first carts started at the same speed in both demonstrations, explain using momentum why the inelastic collision ended slower than the elastic collision.
Practice problems for elastic collision:
- A 4.00 kg metal cart is sitting at rest on a frictionless ice surface. Another metal cart whose mass is 1.00 kg is fired at the cart and strikes it in a one-dimensional elastic collision. If the original velocity of the second cart was 2.00 m/s, what are the velocities of the two carts after the collision?
- Identify the following collisions as most likely elastic or most likely inelastic.
- A ball of modeling clay dropped on the floor.
- A fender-bender automobile collision.
- A golf ball landing on the green.
- Two billiard balls colliding on a billiard table.
- A collision between two ball bearings.