Question

A plastic cube with a coin taped to its top surface is floating partially submerged in water. A student marks the level of the water on the cube, and then removes the coin and tapes it to the bottom of the cube. Afterward, the student returns the cube to the water. The mark the student placed on the cube now sits___.
(A) above the water line
(B) below the water line
(C) level with water line

Answer 1

Answer:

(A) above the water line

Explanation:

As we know that when cube + coin system is floating in the water then the net force due to buoyancy is balanced by the weight of the object

so we will have

$mg = \rho V g$

so here we know that

V = displaced volume of the liquid

$V = \frac{m}{\rho}$

now when coin is placed at the bottom of the cube then the total mass is unchanged

so we have

$V = \frac{m}{\rho}$

so the volume displaced will be same

but initial total volume is displaced by the cube only after coin is placed downwards then the same volume is displaced by cube + coin both

so here the mark will above the water line