A rock of mass M with a density twice that of water is sitting on the bottom of an aquarium tank filled with water. The normal force exerted on the rock by the bottom of the tank is
A) 2Mg.
B) Mg.
C) Mg/2.
D) zero.
E) 3Mg.

Question

Quizplus · Accepted Answer

The rock experiences a buoyant force equal to the weight of the water it displaces. Since the rock's density is twice that of water, the buoyant force is Mg/2. The normal force is the weight of the rock minus the buoyant force, which is Mg - Mg/2 = Mg/2.

A Rock of Mass M with a Density Twice That