Question

Please solve q 74?

Answer 1

See below

Explanation:

There are ways to do this that can remove doubt about the intuition/shortcuts that follows, but I doubt they are appropriate for a multiple choice question.

If we look at the start point, we have a 1 x 1 x 1 cube of water that we can model as a point mass with potential energy `U_1` equal to:

• `U = mgh = m g *1/2`

By pumping all of the water out - assuming that means pumping through the top of the cube such that each and every water molecule now resides at height `h = 1 m` - the body of water will have potential energy:

• `U_2 = mg * 1`

This requires work `W`, where:

`W = U_2 - U_1`

`= (mg)/2 approx (1000 * 10)/2 = 5000 \ J`