Question

A small disc of mass $m$ is placed on a body of mass $M$ at rest. If the disc is set in horizontal motion with velocity $v$ as shown,

(Assume that disc does not leave surface of $M$ and all surfaces are smooth)

• A. the block $M$ will start moving due to force exerted by $m$
• B. the block $M$ stays at rest as surface is smooth
• C. block $M$ and $m$ always move with same speed
• D. block $M$ and $m$ can never move with same speed

Answer 1

The correct option is A the block $M$ will start moving due to force exerted by $m$

The block $M$ will start moving right, due to normal reaction exerted on it by $m$.
Option $a$ is correct.
As there is no external force acting on system, net momentum is conserved.
Initially disc has velocity $v$ and $M$ is at rest.
When disc $m$ reaches maximum height on $M$ ie, $m$ is at rest wrt $M$
let common velocity of masses be $V$
then,
$mv=(M+m)V$
Hence, blocks $m,M$ can have same velocity, when disc $m$ reaches maximum height, before that their speeds are different.
$c,d$ are false.