A small block of mass `M` move on a frictionless surface of an inclimed from as down is figure . The engle of the inclime suddenly change from `60^(@)` to `30^(@)` at point `B` . The block is initally at rest at `A` Assume the collsion between the block and the incline are totally inclassic `(g = 10m//s^(2)`)

If collision between the block and the incline is completely elestic , then the vartical (apward) component of the of the block at point `B` immediatly after it stricess the scond indine is -

Options

A. `sqrt(30) m//s`
B. `sqrt(15) m//s`
C. `0`
D. ` - sqrt(15) m//s`

Answer

C

Explanation

The velocity of the along `BC` just before collision is `v cos 30^(@)` The impact force act perpendicular to the surface so the component of velocity along the increase remains conchanged .

Also since the collision is elastic , the veritical compnment remaining the same as shown in the figure ,so the recungular componants of velocity ofter collision are as shown in the figure the means that the final velocity of the block should be horizentqal making as angle `30^(@)` with BC` Therefore the veritical component ofthe velocity of the block is zero