Hooke’s law Hooke’s law states that the extension of a spring force is proportional to the force used to stretch the spring. F ∝ x ‘Proportional’ means that if the force is doubled then the extension also doubles. The line on a graph of force against extension will be a straight AND go through the origin.

F = k x Springs  the change in its length the force required to stretch it the change in its length  F = k x k=the spring constant

Force against extension graph Force (N) Extension (mm) 6

Does a larger k value mean that a spring is easier to stretch, or harder to stretch?

The right hand spring has been stretched beyond its elastic limit Up to a certain extension if the force is removed the spring will return to its original length. The spring is behaving elastically. If this critical extension is exceeded, known as the elastic limit, the spring will be permanently stretched. Hooke’s law is no longer obeyed by the spring if its elastic limit is exceeded. The right hand spring has been stretched beyond its elastic limit

Force against extension graph if the elastic limit is exceeded Force (N) Extension (mm) elastic limit 9

How far does it stretch if you suspend a 2 N weight from it instead? Ex: If a spring stretches by 20 cm when you pull horizontally on it with a force of 2 N, what is its spring constant? 2 N How far does it stretch if you suspend a 2 N weight from it instead? 2 N

Hooke’s Law lab spring weights pin metre rule SPP (Short Sharp Practical) Aim: Determine the spring constant (k) for a single spring by finding the gradient from a graph of F (N) vs x (m). Use masses 50g to 250g, let g = 10ms-2 11

Choose appropriate words to fill in the gaps below: Hooke’s law states that when a wire or spring is _________ the increase in length or _________ is proportional to the load ______ applied. This law is not obeyed if the spring is taken beyond its ______ limit after which it will become _____________ stretched. A ________ band does not obey Hooke’s law. A graph illustrating Hooke’s law will have a line that is ___________ and passes through the _______. stretched extension force elastic permanently rubber straight origin WORD SELECTION: stretched elastic permanently extension rubber origin force straight 12

Spring Potential Energy, Ep x F = kx F x kx Work