Adock worker pulls two boxes connected by a rope on a horizontal floor, as shown in the figure . all the ropes are horizontal, and there is some friction with the floor. make a free-body diagram of the box b.

The worker pulls the boxes. The force (F) on box B acts towards right direction. The weigh (mg) of the box acts in the downward direction where as reaction force -Normal force (N) acts in the upward direction.

The contact surface between box  and ground is not smooth and hence friction (f ) would act in the opposite direction of the motion of the box. Tension (T) exist in the rope connecting the two boxes.

The free body diagram of box B is attached below showing all the forces acting on it.

no. dot product   =0 when vecs at right angles