forces and elasticity

• Forces are responsible for changing the motion of objects
• If more than one force is present, the shape of an object can also be changed
• Force


  A push or a pull. The unit of force is the newton (N).
• Change of shape


  • Bending an object's ends past each other
  • Pulling an object's ends apart
  • Pushing an object's ends together
• Deformation


  Changing shape and/or size as a result of forces being applied
• Elastic deformation


  Elastic materials return to their original shape and size after being stretched or squashed
• Inelastic deformation


  Inelastic materials do not return to their original shape and size after being stretched or squashed
• Elastic deformation


  • A rubber band stretched a little
• Inelastic deformation


  • A metal drinks can squashed
• Hooke's law describes the relationship between the force applied to a spring and the extension of the spring
• Energy is stored in a spring when it is stretched or compressed
• Force


  Responsible for changing the motion of objects
• Hooke's law


  Force = spring constant x extension (F = k e)
• Hooke's law


  • Force (F) is measured in newtons (N)
  • Spring constant (k) is measured in newtons per metre (N/m)
  • Extension (e), or increase in length, is measured in metres (m)
• Calculating spring constant (k)

  1. Rearrange F = k e to k = F/e

  2. Plug in values: k = 3 N / 0.15 m

  3. k = 20 N/m
• Limit of proportionality


  The point beyond which Hooke's law is no longer true when stretching a material
• Spring constant (k)


  Measure of the stiffness of a spring up to its limit of proportionality
• Proportional


  When two quantities have the same ratio or relative size
• Elastic limit


  The furthest point a material can be stretched or deformed while being able to return to its previous shape
• Inelastic


  Deformation where the object does not return to its original length or shape when the force is removed
• Exceeding the elastic limit


  The relationship between force and extension changes from being linear to non-linear
• Materials showing non-linear extension

  • Clay
  • Putty
• Force-extension graphs


  • Linear extension and elastic deformation can be seen below the limit of proportionality
  • Non-linear extension and inelastic deformation can be seen above the limit of proportionality
• Gradient of force-extension graph before limit of proportionality


  Equal to the spring constant
• Forces


  Responsible for changing the motion of objects
• Hooke's law


  Describes the relationship between the force applied to a spring and the resulting extension or compression
• Elastic potential energy


  Energy stored in squashed, stretched or twisted materials
• Energy stored in a spring

  1. Work done = force × distance moved in the direction of the force

  2. Elastic potential energy = 0.5 × spring constant × (extension)^2
• Extension


  • Increase in length, for example, as a result of being pulled
• Compression


  • Shortening in length, for example, as a result of being squeezed
• Elastic potential energy = 0.5 × spring constant × (extension)^2
• This equation also works for the reduction in length when a spring is compressed
• Forces are responsible for changing the motion of objects. If more than one force is present, the shape of an object can also be changed.
• Practical - forces affect the extension of spring

  1. Secure a clamp stand to the bench

  2. Use bosses to attach two clamps to the clamp stand

  3. Attach the spring to the top clamp, and a ruler to the bottom clamp

  4. Adjust the ruler so that it is vertical, and with its zero level with the top of the spring

  5. Measure and record the unloaded length of the spring

  6. Hang a 100 g slotted mass carrier - weight 0.98 newtons (N) - from the spring. Measure and record the new length of the spring

  7. Add a 100 g slotted mass to the carrier. Measure and record the new length of the spring

  8. Repeat until you have added a total of 1,000 g
• Extension


  Length - unloaded length
• Force and extension for a spring

  As force increases, extension increases
• It is important to keep the ruler vertical
• Wear eye protection. Support and gently lower masses whilst loading the spring.
• Gently lower load onto spring and step back.