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A small block of mass M moves on a fric...

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

A

`sqrt(30) m//s`

B

`sqrt(15) m//s`

C

0

D

`-sqrt(15) m//s`

Text Solution

Verified by Experts

The correct Answer is:
C

In elastic collision, component of `v_(1)` parallel to `BC` will remailn unchanged, while component perpendicular to `BC` will remain unchanged in magnitude but its direction will be reversed.

`v_(||) =v_(1)cos30^(@)=(sqrt60)(1/2)=sqrt(15)m//s`
Now the vertical component of velocity of the block
`v'=v_(_|_)cos30^(@)-v_(||)cos60^(@)`
`=(sqrt(15))((sqrt(3))/2)-(sqrt(45))(1/2)=0`
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