Question

Find the direction of friction on $A$ and $B$ respectively at the common surface between blocks $A$ and $B$ for the figure shown.

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

Answer 1

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

Friction opposes the relative motion and has nothing to do with forces unless they are at rest.
Hence, $v_{AB}=2\hat{i}-\left(4\hat{i}\right)=-2\hat{i}m/s$.
For $B$, $A$ appears to move backwards. Hence friction will be acting in forward direction on $A$ and in backward direction on $B$.
The FBDs can be shown as