# Hookes Law n Hookes Law gives the relationship

• Slides: 13

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

How different materials behave n n n 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 the force affects the extension.

Elastic and wire n Different materials react differently when a force is applied to them.

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

n n Diagram The apparatus is set up as shown. For the purposes of this experiment we shall be using loads of 100 g, and the extension of the spring shall be measured in metres. Extension = present length – original length

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 Total Hangi ng Mass (g) Total Hangi ng Mass (kg) Equilibrium length _____m Total force (mg) g= 10 N/kg Stretched length (m) Extension (m) 100 200 300 400 500 600 700 800 900 1000 1600

n n n Graph: Plot a graph of force against extension. n Force (N) n n n Extension (m) 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?

Hooke’s Law n n "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.

Elastic Limit tic be ha vio ur Plastic behaviour as n 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. El n Elastic limit

Repeat the experiment using an elastic What do you notice? n Does an elastic obey Hooke’s Law? n

Class Experiment – Stretching a wire – Vernier Scale n 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.

F copper Elastic and wire n n F e rubber n e 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.

F rubber Hysteresis n n n 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?