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. `m//5`
B. `2m//5`
C. `3m//5`
D. `4m//5`

Answer

B

Explanation

Let force applied by water on box is F

`F-mg=m(g)/(6)" ….(1)"`
`(M+m)g-F=(M+m)(g)/(6)" ….(2)"`
add (1) and (2)
`Mg=(Mg)/(6)+(2mg)/(6) rArr M=(2m)/(5)`