JAMB Physics 2021 Paper

Concept:Archimedes' principle explains that a solid fully immersed in a fluid loses apparent weight equal to the weight of the fluid displaced.The relative density (R.D) of a liquid is obtained by comparing this loss of weight in the liquid with the loss of weight in water.Explanation:Loss of weight in water $= 10.00\,\text{N} - 6\,\text{N} = 4\,\text{N}$.Loss of weight in liquid X $= 10.00\,\text{N} - 7\,\text{N} = 3\,\text{N}$.Since the solid is fully immersed in both fluids, the volume displaced in liquid X is equal to the volume displaced in water.Therefore, R.D of liquid X $= \frac{\text{loss of weight in liquid X}}{\text{loss of weight in water}} = \frac{3}{4}$.This is a dimensionless ratio, so the relative density of liquid X is $\frac{3}{4}$.Answer:Relative density of liquid X $= \frac{3}{4}$, which is option C.