Question

An elevator platform is going up at a speed `20 ms^(-1)` and during its upward motion a small ball of `50 g` mass falling in downward direction strikes the platform elastically at a speed `5 ms^(-1)`. Find the speed (in `ms^(-1)`) with which the ball rebounds.

Answer 1

Correct Answer - 4,5
The situation is analysed in the figure. We casn consider mass of platform to be very large compared to that of ball, so we have

`vecv_(1)=vecu_(1)e=((v_(2)-v_(1))/(u_(2)-u_(1)))=((v_(2)-u_(1))/(u_(1)-(-u_(2))))impliesvecv_(2)=2vecu_(1)-vecu_(2)`
Thus rebound velocity of ball is
`vecv_(2)=2xx20-(-5)=45ms^(-1)` upward