Learning Objective Describe Hookes Law and calculate force To be able to: Describe Hookes Law and calculate force Key Words: Spring, force, extension

Exam question Saturday, 25 May 2019 Hookes Law Date and title in books. You need a pen, pencil & ruler. Starter Activity – 5 minutes Exam question

(F) Target 1-9 State Hookes Law in works and state the equation to find the force on a spring (F/H) Target 4-9 Calculate the force causing a spring to extend (H) Target 5-9 Rearrange the equation to calculate spring constant and extension Extension: Application of Knowledge Target 8/9 Explain the consequences of over-stretching a spring Learning Outcomes

Stretching a Spring A stretched spring exerts a pull force on the object holding each end of the spring. This pull, which we call the tension, is equal and opposite to the force needed to stretch the spring. The more a spring is stretched, the more tension in it.

Practical Activity Set up a clamp and stand with a spring. Measure the spring length with no load (0N) Add 100g (=1N) to the spring and record the length. Do this up to 10N (hopefully!) Record your results in a table.

Force (N) Length (mm) Extension (mm) 1 2 3 4 5 6 7 8 9 10

Graph Use your results to create a graph! Plot x = load, y = spring extension What do you notice?

Our Results Your results should give a straight line through the origin (hopefully!!) From this we can deduce: The force needed to stretch a spring is proportional to the extension of the spring This is Hooke’s Law!

Hooke’s Law Hookes Law states: The force needed to stretch a spring is directly proportional to the extension of the spring from its natural length F = k e Where F is the force (or load) in Newtons, k is the spring constant in Nm-1 and e is the extension from its natural length in m

The Graph F = k e So a graph of force against the extension (F against e) is a straight line with a gradient k. The greater the value of k, the greater the stiffness of the spring. The unit of k is Nm-1.

Elastic Limit Eventually, the spring will deform permanently. It has reached its elastic limit – on a graph this shows us as a curve at the end of the straight line.

limit of proportionality Results plastic region force (N) break point limit of proportionality elastic limit elastic region extension (cm) If a spring is stretched far enough, it reaches the limit of proportionality and then the elastic limit. Teacher notes It should be pointed out that the limit of proportionality is at the point where the graph is no longer a straight line. Students should be able to make sense of this since a straight line graph represents direct proportionality. The link between the definitions of elasticity and elastic limit could be highlighted – the elastic limit is the extent to which a material displays the property of elasticity. The elastic limit is a point beyond which the spring will no longer return to its original shape when the force is removed.

Hooke’s law The limit of proportionality is a point beyond which behaviour of an elastic material no longer conforms to Hooke’s law: The extension of a spring is directly proportional to the force applied, provided its limit of proportionality is not exceeded. F µ e or F = ke where k is a constant. original length x Teacher notes F is the force in newtons, N; k is the spring constant in newtons per metre, N/m; e is the extension in metres, m. The stiffer a spring is, the greater its spring constant (k). F