Question

In the following figure, find the direction of friction on the blocks $A$ and $B$ and the ground respectively.

• A. $\text{Backwards, forwards, backwards}$
• B. $\text{Backwards, backwards, forwards}$
• C. $\text{Forwards, forwards, backwards}$
• D. $\text{Forwards, backwards, forwards}$

Answer 1

The correct option is A $\text{Backwards, forwards, backwards}$

Direction of kinetic friction depends on relative velocity, not on the force.
So, velocity of block A w.r.t block B,
$v_{AB}=-3\hat{i}-(-6\hat{i})=3\hat{i}m/s$
For $B$, $A$ appears to be moving forwards.
Hence, friction will be acting in backward direction on $A$ and in forward direction on $B$ at the interacting surface $AB$.
Now, velocity of block B w.r.t ground,
$v_{BG}=-6\hat{i}-0=-6\hat{i}$
For ground, $B$ appears to move backwards.
Hence, friction will be acting in forward direction on $B$ and in backward direction on the ground.
Now, the FBD can be as shown