Question

Two blocks $A$ and $B$ of masses $m_A$ and $m_B$ have velocity $v$ and $2v$, respectively, at a a given instant. A horizontal force $F$ acts on the block $A$. There is no friction between ground and block $B$ and coefficient of friction between $A$ and $B$ is $\mu$. The friction

• A. on $A,$ supports its motion.
• B. on $B,$ opposes its motion relative to $A$.
• C. on $B,$ opposes its motion.
• D. opposes the motion of both.

Answer 1

Answer: (A), (B), (C)

The correct options are
A on $A,$ supports its motion.
B on $B,$ opposes its motion relative to $A$.
C on $B,$ opposes its motion.

As friction opposes relative motion between A and B, it acts as shown in figure.

From the figure it is clear that friction on $A$ supports its motion and on $B$ opposes its motion.
Friction always opposes relative motion.