A force acting on an object may change its shape, size or direction. An elastic object regains its original shape when the forces deforming it are removed.
Experiment
The elasticity of an object can be measured by hanging weight form it.
1. The strip of material to be tested is clamped at its upper hand. A weight hanger is attached to the material to keep it straight. 2. The length of the strip is measured using a metre ruler. This is its original length. 3. The weight hung from the material is increased by adding weights one at a time. The strip stretches each time more weight is hung from it. 4. The length of the strip is measured each time a weight is added. The total weight added and the total length of the strip are recorded and displayed by an extensionload graph.
The increase of length from the original is called the extension. This is calculated each time a weight is added and recorded:
extension of material at any stage = its length at the stage – its original length
Elastic energy
When an elastic object is stretched, elastic potential energy is stored in the object. This is because work is done on the object by the force. When the force is removed, the stored energy is released. Some of this energy may be transferred into kinetic energy of the object or make its atoms vibrate more making the object warmer.
Hooke’s Law
The extension of a spring is directly proportional to the force applied, provided its limit of proportionality is not exceeded.
If the extension of any stretched object or material is directly proportional to the stretching force, it obeys Hooke’s Law. If the force is too large the spring stretches more than predicted. This is because the spring has been stretched beyond its limit of proportionality .
Hooke’s Law may be written as an equation:
Force applied (N) = spring constant (N/m) × extension (m)
F = k × e, where: F = force in newtons, N
k = spring constant in newtons per metre , N/m
e = extension in metres, m
The spring constant of a spring is the force per unit extension needed to stretch it, assuming it limit of proportionality is not reached.