Changing Shape (OCR GCSE Physics A (Gateway))

Forces & Changing Shape

• 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

Elastic & Plastic Deformation

• 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