We all know when we put the rolled up piece of paper at the end of a rubber band after stretching it, it will shoot to quite a distance upon release. On releasing the rubber band, it goes back to its former relaxed shape. This property of the rubber band is known as elasticity.
In essence, elasticity is the property of any object by virtue of which it regains its shape when the forces that were distorting the shape in the first place are removed. A trampoline is also made up of an elastic surface. When you jump on it, you are distorting the shape of the surface which has been bound to the frame of the trampoline to make it taut. The elastic surface resists this change in its shape and tries to regain its former flat and taut surface. This causes you to jump higher and higher as the trampoline tries to shake off the external force that you were applying.
But you cannot keep on stretching the rubber band infinitely and hope that it will gain its shape back every time. After stretching it quite a bit, the rubber band snaps and breaks. This happens because there is a limit up to which the object exhibits the property of elasticity. If a force is applied which exceeds this threshold, the object starts to yield. “To yield” means its shape and size changes irreversibly and the object does not go back to its former shape. If the rubber band is stretched still, there is a breaking point after which the rubber band starts to become thinner and thinner and eventually breaks.
An elastic deformation, as discussed above, is a deformation in which the objects regain their shape after the external force is removed. On the other hand, a plastic deformation is one in which the object experiences a permanent change in their shape or size or both. The bending of a straight pipe by a strongman in order to demonstrate his strength is an example of a plastic deformation. The pipe can be bended and beaten back to its former shape but it will not achieve its former straight shape on its own.
Why does an elastic object regain its shape anyway? Why doesn’t it experience a permanent deformation? Whenever an external force is applied to an elastic object, it experiences an internal restoring force which acts in the opposite direction. The more the object is stretched, the bigger is the restoring force that acts in the direction opposite to that of the external force. This proportionality is known has Hooke’s law.
Dennis the Menace
When Dennis stretches his slingshot, he is supplying an external force. The stretching band of the slingshot is made out of rubber which is elastic in nature. On being stretched, a restoring force acts on the band and the arms of the slingshot try to regain their shape. In doing so, the stones are catapulted at Dennis the Menace’s targets and he has to be reprimanded for making use of physics in an uncontrolled but fun way.