Hooke

1
Hookes Law

• Hooke's Law gives the relationship between the
  force applied to an unstretched spring and the
  amount the spring is stretched.

2
How different materials behave

• Recall How does a spring stretch when a force is
  applied to it?
• Try stretching a piece of thin copper wire and an
  elastic.
• How do they differ from the springs?
• What does it feel like as you pull harder and
  harder?
• Can you sketch a graph to show how the force
  affects the extension.

3
Elastic and wire

• Different materials react differently when a
  force is applied to them.

4
How does a spring behave?

• Aim We shall conduct an experiment to determine
  how the extension of a spring varies with the
  stretching force.
• A spring is hung vertically from a fixed point
  and a force is applied in stages by hanging
  weights from the spring.

5
Extension  present length  original length

• Diagram
• The apparatus is set up as shown. For the
  purposes of this experiment we shall be using
  loads of 100g, and the extension of the spring
  shall be measured in metres.

6

• Table

• Method
• What is the independent variable? (range?)
• What is the dependent variable? ( How will this
  be measured accurately?)
• What are the control variables?

single spring single spring Equilibrium length __________m
Total Hanging Mass (g) Total Hanging Mass (kg) Total force (mg) g 10 N/kg Stretched length (m) Extension (m)
100      
200      
300      
400      
500      
600      
700      
800      
900      
1000
1600      
 
7

• Conclusion
• Comment on the shape of the best fit line, try to
  describe the pattern which appears. Have you
  found any simple rule for springs?
• What happened to the stretch when you doubled the
  load? And three times?
• Can you work out the gradient? What does this
  gradient mean?
• What happens when large loads are added to the
  spring?
• How would the plot look if you replaced the
  spring with a stiffer spring? weaker spring?

• Graph Plot a graph of force against extension.

Force (N)
Extension (m)
8
Hookes Law

• "Hooke's Law" is about stretching springs and
  wires.
• Hooke's Law states- the extension is
  proportional to the force
• the spring will go back to its original length
  when the force is removed
• so long as we don't exceed the elastic limit.

9
Elastic Limit
Elastic limit

• Below the elastic limit, we say that the spring
  is showing "elastic behaviour" the extension is
  proportional to the force, and it'll go back to
  it's original length when we remove the force.
• Beyond the elastic limit, we say that it shows
  "plastic behaviour". This means that when a force
  is applied to deform the shape, it stays deformed
  when the force is removed.

Plastic behaviour
Elastic behaviour
10
Repeat the experiment using an elastic

• What do you notice?
• Does an elastic obey Hookes Law?

11
Class Experiment Stretching a wire Vernier
Scale

• Two wires of the same material are suspended side
  by side from the same support. The main scale is
  kept taut by the weight L. The extension of the
  wire for different loads is obtained from the
  vernier.

12
Elastic and wire
copper
F

• Different materials react differently when a
  force is applied to them.
• If a material obeys Hooke's Law, its extension is
  proportional to the applied force. If the force
  is removed, the material returns to its original
  length.
• Springs and metal wire obey Hooke's law up to the
  elastic limit. Beyond this point, they are
  permanently deformed. They will not return to its
  original length when the force is removed.

rubber
e
F
e
13
Hysteresis
rubber
F

• What do you notice about the plot when you load
  and unload an elastic?
• What does the area under a graph represent?
• ENERGY!!!
• See for yourself!! Take an elastic and
  repeatedly stretch the elastic while it is in
  contact with your top lip.
• What do you notice?
• How could this energy be measured from the graph?