Hooke's law and force-extension graphsEdexcel International A Level Physics: Mind map

Exam questions on Hooke's law and force-extension graphs

1. A student investigates a helical steel spring hung from a clamp. Masses are added one at a time to the lower end and the extension of the spring is measured after each addition. The spring obeys Hooke's law for all loads up to 8.0 N, at which load its extension is 0.20 m.
   The student adds further masses so that the force is 12 N. She finds that the extension is greater than the value predicted using the stiffness found from the first results. Explain this observation.2 marks
2. Spring X has a stiffness of 60 N m⁻¹ and spring Y has a stiffness of 20 N m⁻¹. Both springs obey Hooke's law for all the forces used in a test.
   A force of 3.0 N is applied to spring Y. Calculate the force that must be applied to spring X to produce the same extension as in spring Y.2 marks
3. A technician loads a metal wire, increasing the force steadily from zero. Up to 24 N the extension is directly proportional to the force, and at 24 N the extension is 1.2 mm. Between 24 N and 30 N the extension increases by larger amounts for each newton added, but the wire still returns to its original length when unloaded. Beyond 30 N the wire no longer returns to its original length when unloaded.
   Describe how the behaviour of the wire changes as the force is increased from zero to 30 N and beyond, using the terms limit of proportionality, elastic limit and plastic deformation.3 marks