We have seen that forces can change the shape of things. But sometimes the changes are not always permanent. A rubber band will stretch, but as soon as you let go it will return to its original shape. As it does so, it exerts a force — called an ‘elastic force’. Metals are harder to stretch but they can exert a greater elastic force. Metals are often coiled to make springs —to be used for machinery parts or trampolines, or to make seats and mattresses more comfortable for example. Elastic forces are also used to absorb a large force — like breaking the fall of a bungee jumper.
When a rubber ball is dropped onto the ground, it is squashed. The ground exerts a force which pushes the ball upwards and back into the air. The ball then returns to its original shape.
As you stretch or compress an elastic material like a bungee cord, it resists the change in shape. It exerts a counter force in the opposite direction. This force is called elastic force. The farther the material is stretched or compressed, the greater the elastic force becomes. As soon as the stretching or compressing force is released, elastic force causes the material to spring back to its original shape.
After the bungee jumper jumps, he accelerates toward the ground due to gravity. His weight stretches the bungee cord. As the bungee cord stretches, it exerts elastic force upward against the jumper, which slows his descent and brings him to a momentary stop. Then the bungee cord springs back to its original shape and the jumper bounces upward.
Bedsprings provide springy support beneath a mattress. The spring in a door closer pulls the door shut. The spring in a retractable ballpoint pen retracts the point of the pen. The spring in a pogo stick bounces the rider up off the ground.