An empty plastic box of mass m is found to accelerate up at the rate of g/6 when placed deep inside water. How much sand should be put inside the box so that it may accelerate down at the rate of g/6?

Options

A. 1.5 kg
B. 2 kg
C. 2.5 kg
D. 4 kg

Answer

B

Explanation

Let F be the upward thrust of water on plastic box. As per question, `F - 5g =5 g//6` …..(i)
Let m be the mass put in the box so that box moves down with ac celeration `g//6`. Then
`(5 + m)g- F=(5+ m)g//6` …..(ii)
Adding (i) and (ii), we have
`mg=(g)/(6)(10+m)` or `6m=10+m`
or `m=2 kg`