Changing Shape

• 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