Change of shape

When a force acts on an object, the object may change shape by bending, stretching or compressing - or a combination of all three shape changes. However, to change the shape of a stationary object there must be more than one force acting to do one of the following.

Bend an object's ends past each other, eg when an archer pulls an arrow back against a bow.

A beam is bent.Two equal forces act inwards to support left hand end. A third force bends the right had end downwards.

Pull an object's ends apart, eg when a rubber band is stretched.

A beam is stretched by two equal forces. Beam is thinner in the middle to indicate stretching. Arrows at each end indicate direction of force.

Push an object's ends together, eg when an empty drinks can is squashed.

A beam is squashed by two equal forces. Beam is thicker in the middle to indicate compression. Arrows either end point inwards to indicate direction of force.

A change in shape is called deformation:

  • elastic deformation is reversed when the force is removed
  • plastic deformation is not fully reversed when the force is removed - there is a permanent change in shape

A rubber band undergoes elastic deformation when stretched a little. A metal drinks can undergoes plastic deformation when it is squashed.