Topic: Dynamics
Concepts: Forces in mechanics, Limiting cases, Newton’s laws

---

Solution:

Since the box is moving at constant velocity, we balance forces in the vertical direction and horizontal direction:

When the block is far away, $\theta \rightarrow 0$ so we have

When the block is close to the pulley, $\theta \rightarrow \pi/2$ so we have

Since $f=0$,

We now go through the possible choices:

• A) Not correct since $T$ changes from $\mu mg$ to $mg$.
• B) Correct since $f$ decreases from $\mu mg$ to $0$.
• C) Not correct since $N$ decreases from $mg$ to $0$.
• D) Not correct for the same reason as C.
• E) Not correct for the same reason as C.

so the answer is B.