Deformation and Hooke’s Law


When a force is applied to elastic objects, they can change shape. This is deformation.

  • When an elastic object is pulled, it stretches
  • When an elastic object is squashed, it compresses

The greater the force, the greater the amount of deformation. This is why a rubber band stretches out further the more you pull it. However, after applying too much force, the object may not return to its original shape.

Hooke’s Law

Hooke’s Law states that the extension of an elastic object is directly proportional to the force applied to it. So, if the force applied to an object is doubled, then the extension of the object also doubles. Also, if there is no force applied, then there is no extension.

To investigate Hooke’s Law, set up the apparatus involving a spring as shown in the diagram below.

1. Set up the above apparatus, including the mass carrier attached to the spring.

2. Attach an empty mass carrier to the bottom end of the spring and measure the length of the spring.

3. Continue adding slotted masses, measuring the spring length after each additional weight is added.

After each slotted mass is added, measure the extension using the formula:

Extension = (Length of spring before adding mass) – (Length of spring after adding mass)

After this, you can plot the extensions on a force-extension graph, with force on the vertical (y) axis and extension on the horizontal (x) axis.

This graph shows us that the force applied is directly proportional to the extension of the object, represented by a straight line passing through the origin (0,0).

As the amount of weight added to the end of the spring increases, the length of the spring is recorded.

Interpreting a force-extension graph

The steeper the force-extension line, the stiffer the spring, and vice versa.

The area under the force-extension curve represents the work done. This energy, required to stretch the spring, is measured in joules (J).

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