A small block of mass `M` moves on a friction-less surface of an inclined plane, as shown in the figure. The angle of the incline suddenly changes from `60^(@)` to `30^(@)` at point `B`. The block is initially at rest at `A`.
If collision between the block and the incline is completely elastic, then the vertical (upward) component of the velocity of the blocks at point `B`, immediately after it strikes the second incline is

Options

A. `sqrt30` m/s
B. `sqrt15` m/s
C. 0
D. `-sqrt(15)` m/s

Answer

C

Explanation

The velocity of the block coming down from the incline AB makes an angle `30^@` with the incline BC. If the block collides with the incline BC elastically, the angle of velocity of the block after collision with the incline shall be `30^@`
Hence just after collision with incline BC the velocity of block shall be horizontal. So immediately after the block strikes the second incline, its vertical component of velocity will be zero.