Changing Shape (Edexcel GCSE Physics: Combined Science)

• For stationary objects, multiple forces have to be applied to change their shape

• Objects can change their shape by:

  • Stretching 
  • Bending 
  • Compressing 

• A combination of all three shape changes can also occur

Compression

• An object is compressed when forces act
  • In opposite directions
  • Towards the object
• An example of compression is placing a mass on top of a spring placed on a flat surface
• The two forces are:
  • The weight of the mass on the spring 
  • The reaction force of the surface on the spring

The compression or stretching of a spring requires two forces

Stretching

• An object is stretched when forces act:
  • In opposite directions
  • Away from the object
• An example of stretching is placing a mass on the bottom of a spring hanging vertically from a rod
• The two forces are:
  • The weight of the mass of the mass on the spring
  • The tension in the spring

• These two opposing forces cause a tension force in the spring

Bending

• An object is bent when multiple forces act:
  • In opposing directions
  • At different points on the object
• An example of bending is a diving board bending when a swimmer stands at the far end
• The two forces are:
  • The weight of the swimmer on the diving board
  • The reaction force of the block on the dividing board

Forces on a diving board cause it to be bend when a swimmer stands on one end

More than one force on an object can cause it to bend

• When some objects, such as springs or rubber bands, are stretched they will return to their original shape and length once the forces are removed
  • Other materials, such as plastic, remain permanently distorted (stretched)

Elastic materials return to their original shape and size after stretching whilst plastic materials don’t

• A change of shape is called a distortion and can either be:
  • Elastic
  • Inelastic

Elastic Distortion

• Elastic distortion occurs:

When objects return to their original shape when the stretching force is removed

• Examples of materials that undergo elastic distortion are:
  • Rubber bands
  • Fabrics
  • Steel springs

Inelastic Distortion

• Inelastic distortion occurs:

When objects remain stretched and do not return completely to their original shape even when the stretching force is removed

• Examples of materials that undergo inelastic distortion are:
  • Plastic
  • Clay
  • Glass