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JAMB Physics 1978 Question 14

Question 14

Density and Upthrust

A solid suspended by a piece of string is completely immersed in water. On attempting to lift the solid out of the water, the string breaks when the solid is partly out of the water. This is because

  1. A.the tension in the string decreases as the solid is lifted
  2. B.the mass of the solid has increased
  3. C.the solid apparently weighs less when completely immersed in water than when partially immersed
  4. D.part of the solid still in water is exerting more force on the string
  5. E.the solid is no longer in equilibrium with water.
Show the answer and explanation

Answer: C (the solid apparently weighs less when completely immersed in water than when partially immersed)

Explanation

As the solid comes out of the water, less water is displaced, so the upthrust falls and the string must hold more of the weight. The tension is greatest when the solid is partly out, so the string breaks then.