JAMB Physics 2018 Paper

Concept:Loss of weight in a fluid equals the upthrust, which is the weight of fluid displaced.The volume of the object stays the same in every liquid.Explanation:In water, upthrust = weight in air − weight in water.Upthrust = $30\text{ N} - 21\text{ N} = 9\text{ N}$.Mass of water displaced = $\frac{9\text{ N}}{10\text{ m/s}^2} = 0.9\text{ kg}$.Volume of object = volume of water displaced = $\frac{0.9\text{ kg}}{1000\text{ kg/m}^3} = 9 \times 10^{-4}\text{ m}^3$.The given liquid has relative density $1.4$, so its density is $1.4 \times 10^3\text{ kg/m}^3$.Mass of liquid displaced = density × volume.Mass = $(1.4 \times 10^3)(9 \times 10^{-4}) = 1.26\text{ kg}$.Weight of liquid displaced = $1.26 \times 10 = 12.6\text{ N}$.Therefore, apparent weight in the liquid = $30\text{ N} - 12.6\text{ N} = 17.4\text{ N}$.Answer:B. $17.4\text{ N}$