(a)

The density of water is .

Acceleration due to gravity

The side of the cube .

Depth of the cube at the top is .

The area of the cube is .

Hydrostatic force : .

Substitute corresponding values in the above.

.

The top of the cube is .

(b)

The water level is deep.

Here .

Differentiate with respect to .

.

The depth of the rectangle is .

Average depth of a vertical side .

Substitute corresponding values in the above.

Apply integration to the above equation.

Substitute in the above equation.

One of the sides of the cube .

(a) The top of the cube is .

(b) One of the sides of the cube .