A rock of mass M with a density four times that of water is

A rock of mass M with a density four times that of water is resting on the bottom of an aquarium tank filled with water. The normal force exerted directly on the rock by the bottom of the tank is:
A) 3Mg/4
B) 2Mg/3
C) Mg/4
D) Mg/3

Solution

Given density of rock is four times the density of water

p_rock=4p_water

M_rock =p_rockV=4p_waterV

Mass of water

M_water=M_rock/4

Weight of liquid displaced is the weight of the liquid that has been replaced by the rock

N=Mg -(Mg/4) =3Mg/4