Question

In the figure shown, find the direction of friction on blocks B and C at the surface which is common to the blocks B and C respectively.

• A. $\text{Forward, Backward}$
• B. $\text{Backward, Backward}$
• C. $\text{Backward, Forward}$
• D. $\text{Forward, Forward}$

Answer 1

The correct option is A $\text{Forward, Backward}$

Velocity of block B w.r.t block C,
$v_{BC}=2\hat{i}-3\hat{i}=-\hat{i}m/s^2.$
That is for $C$, block $B$ appears to move backwards. Hence, friction will act forward on block $B$ and in backward direction for block $C$. (Since, friction opposes relative motion)
The FBD of the blocks w.r.t common surface $BC$ can be shown as