Changing Shape (OCR Gateway GCSE Physics: Combined Science)

• For stationary objects, more than one force has to be applied to change their shape
• Their shape can change by:
  • Stretching (forces in opposite directions away from the object)
  • Bending (forces that deform the object)
  • Compressing (forces in opposite directions towards the object)

• A combination of all three shape changes can also occur

Compression

• An example of compression is placing a mass on top of a spring resting on a flat surface
• The two forces are:
  • The weight of the mass
  • The reaction force from the surface to the spring

• These two forces act towards each other

The compression or stretching of a spring requires two forces

Stretching

• An example of stretching is placing a mass on the bottom of a vertically hanging spring
• The two forces are:
  • The weight of the mass
  • The tension in the spring

• These two forces act away from each other
  • These opposite forces are a result of Newton's Third Law

Bending

• 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
  • The reaction force from the block to the dividing board

• These two forces act towards each other, but at different points on the object
• Bending can also be caused by two forces at an angle to each other

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 deformed (stretched)

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

• A change of shape is called a deformation and can either be:
  • Elastic
  • Plastic

Elastic Deformation

• Elastic deformation 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

Plastic Deformation

• Plastic deformation 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