Question

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

• A. $\sqrt{30}m/s$
• B. $\sqrt{15}m/s$
• C. $0$
• D. $-\sqrt{15}m/s$

Answer 1

The correct option is C $0$

The velocity of the block coming down from the incline $AB$ makes an angle ${30}^{\circ }$ 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 ${30}^{\circ }$.
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.