Question

There are two observers A and B, observer A is fixed to the smooth plank and
observer B is standing on smooth horizontal ground. The plank is accelerated
with constant acceleration and a block (C) is placed on it then

(A) Momentum of block C is conserved as seen by observer A
(B) Momentum of block C is conserved as seen by observer B
(C) Net force on plank is zero as seen by observer A
(D) Net force on plank is non-zero as seen by observer B​

Answer 1

A is the answer as momentum is conserved in that statement

Answer 2

Answer:

Option A is the right choice.

Explanation:

Momentum of a body (P) is,

                                            $P=mv$

Momentum of a body is conserved when,

                                              $F_{ext} =0\\P= constant$

Observer A is fixed on the plank itself so for him there is no relative acceleration of block C, so there is no external force acting on block C from his frame of reference.

Hence, momentum of block C is conserved as seen by observer A.