A scale from which a rock is suspended reads 5 N when the rock is out of water and 3 N when the rock is submerged. The density of the rock is
A)the density of water.
B)1.5 times the density of water.
C)2.5 times the density of water.
D)3.5 times the density of water.
E)none of the above

Question

Quizplus · Accepted Answer

The difference in weight between when the rock is out of water and when it is submerged is equal to the weight of the water that the rock displaces. Using the formula for density, we have density = mass/volume. Since the mass is constant, the density of the rock is proportional to its volume. The volume of water displaced by the rock is equal to its own volume, so the ratio of the volume of the rock to the volume of water displaced is equal to the specific gravity of the rock. Since the specific gravity is the ratio of the density of the rock to the density of water, and the specific gravity is 2.5, then the density of the rock is 2.5 times the density of water. Therefore, the best choice is C.

A Scale from Which a Rock Is Suspended Reads 5