Changes 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 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 distortion:

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

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