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JAMB Physics 1982 Question 20

Question 20

Density and Upthrust

Why does a piece of cork float on water?

  1. A.There is no upthrust on it when completely immersed in water
  2. B.The upthrust on it when completely immersed in water is greater than its weight in air
  3. C.The upthrust on it when completely immersed in water is less than its weight in air
  4. D.Its specific gravity is greater than that of water
  5. E.Its specific gravity is the same as that of water.
Show the answer and explanation

Answer: B (The upthrust on it when completely immersed in water is greater than its weight in air)

Explanation

Cork is less dense than water, so when fully under water it displaces water weighing more than itself. The upthrust is bigger than its weight and pushes it up until it floats.