WAEC Physics 2012 Theory — Question 2
Question 2 of 15 from the West African Examinations Council (WAEC) Physics 2012 Theory paper, with the correct answer and a full explanation.
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2. A spiral spring with a metal extends by 10.5cm in air. When the metal is fully submerged in water, the spring extends by 6.8cm. Calculate the relative density of the metal. (Assume Hooke's law is obeyed.)
Model answer
Since Hooke's law is obeyed, extension is proportional to the force (weight) producing it. Weight in air ∝ 10.5cm; apparent weight in water ∝ 6.8cm. Upthrust (loss in weight) ∝ (10.5−6.8) = 3.7cm. Relative density of the metal = Weight in air/Upthrust = 10.5/3.7 = 2.84.
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