Question

A balloon is rising up along the axis of a concave mirror of radius of curvature`20m`A ball is dropped from the balloon at a height `15m`from the mirror when the balloob has velocity `20m//s`.Find the speed of the image of the ball formed by concave mirror after`4`seconds[take:`g=10m//s^(2)`]

Answer 1

Correct Answer - `80m//s`

Velocity of ball after`4`sec
`v=u+atrArrv=20-10xx4 rArr=-20m//s`
After `4`sec velocity of ball is`20m//s`towards mirror and position is at`15m`.
`(1)/(v)+(1)/(u)=(1)/(f)rArr(1)/(v)+(1)/(-15)=(1)/(-10)`
`(1)/(v)=-(1)/(10)+(1)/(15)rArr(1)/(v)=(-3+2)/(30)`
Since,`(dv)/(dt)=-((v)/(u))^(2)xx(du)/(dt)rArr(dv)/(dt)=-((-30)/(15))^(2)xx20m//s`
`(dv)/(dt)=-80m//s`