HOOKE'S LAW
4 marks · NormalA student investigates how extension varies with force for a metal wire. Their setup uses a half-metre rule, clamp stand, and a set square to improve measurement accuracy.
Describe two ways the student can ensure the measurements of the wire's extension are accurate in this investigation.
Explain why the spring or wire should not be stretched beyond its elastic limit during this experiment.
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(a) Use a set square to read the extension at eye level and measure from the same reference point each time; ensure the ruler is vertical and not parallax errors.
(b) If stretched beyond the elastic limit, the wire will not return to its original length, so Hooke's law is no longer valid and the results are incorrect.
Common mistakes: Vague references to being careful without details; Confusing elastic and proportional limit
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Marking points
B1 (a) · 1 mark · Use of set square or mention of eliminating parallax error.
B2 (a) · 1 mark · Consistent measuring from the same reference point or ruler vertical.
B3 (b) · 1 mark · Explanation that wire will not return to original length (permanent deformation).
B4 (b) · 1 mark · Link to Hooke's law not being obeyed/invalid results beyond elastic limit.
Worked solution
a) To ensure accurate measurements, the student should use a set square to take extension readings at eye level, avoiding parallax errors, and always measure from the same reference point on the wire each time. Also, check the ruler is vertical and aligned with the wire. b) The spring or wire should not be stretched beyond its elastic limit because, past this point, it will not return to its original length—even when the force is removed. Beyond the elastic limit, Hooke's law does not apply, so any measurement made would not reflect the true elastic behaviour of the material.