Given below are two statements:
Statement I: A block lies on the rough floor of a lift that is falling freely. If the block is given a horizontal push, it moves with constant velocity relative to the lift.
Statement II: In a frame moving with acceleration $\vec a$, every body of mass $m$ experiences a pseudo force $m\vec a$ in the direction of $\vec a$.
In the light of the above statements, choose the correct answer from the options given below:
Answer: (B) Statement I is true but Statement II is false
Concept. In an accelerating frame a pseudo force $-m\vec a$ acts on each body, opposite to the frame's acceleration. Friction depends on the normal force.
Step 1. Statement I. In the lift frame the block feels $mg$ down and a pseudo force $mg$ up, so the floor pushes with $N = 0$. Kinetic friction $\mu N = 0$, and no other horizontal force acts, so the block moves with constant velocity relative to the lift. True.
Step 2. Statement II. The pseudo force is $-m\vec a$, opposite to the frame's acceleration, not along it. False.
Answer. Statement I is true but Statement II is false (Option B).
Solution by Sreeraj P, M.Sc Physics