A ball of mass 1 kg falls from a height of 5m above the free surface of water. The relative density of the solid ball is `s=2//3`. The ball travels a distance of 2m under water and becomes stationary. The work done by the resistive forces of water is

Options

A. `-50 J`
B. `-20 J`
C. `-40 J`
D. `-30 J`

Answer

C

Explanation

From work-energy theorem,
work done by all the forces=change in kinetic energy.
`therefore` (Work done by gravity for 7m)+(work done by upthrust for 2m)+(work done by resistive forces)=0 as `Delta KE =0`
`therefore (1)(10)(7) cos 0^(@)+((1)/(2//3xx rho_(w)))(rho_(w)g)`
(2) cost `80^(@)`+ work done by resistive forces =0
`therefore ` Work done by resistive forces= - 40 J