An object weighs 15 N in air. When fully immersed in water it weighs 12 N, and when fully immersed in another liquid it weighs 13 N. Find (a) the specific gravity (relative density) of the object and (b) the specific gravity of the other liquid.

(a) 5, (b) 2/3
(a) 4, (b) 1/2
(a) 5, (b) 1/3
(a) 4, (b) 2/3

Correct answer: (a) 5, (b) 2/3

Solution

The apparent loss of weight gives the buoyant force. The object's relative density is its air-weight divided by the loss-of-weight in water. The liquid's relative density is the ratio of buoyant forces (same volume displaced).

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