Anita used a single movable pulley to lift a bucket of water from a well. She lubricates the pulley.

Which of the following statements is true regarding the performance of the pulley used?

1. Mechanical Advantage decreases and efficiency increases.
2. Velocity Ratio increases and efficiency decreases.
3. Mechanical Advantage remains unchanged and efficiency increases.
4. Velocity Ratio remains unchanged and efficiency increases.

Velocity Ratio remains unchanged and efficiency increases.

Reason — For an actual single movable pulley, due to (i) friction in the pulley bearings or at the axle, and (ii) the weight of the pulley and string, the effort needed to lift up a load L will be more than $\dfrac {\text L}{2}$, so the mechanical advantage will be less than 2. Since the velocity ratio remains 2, hence the efficiency will always be less than 1 or 100%.

As velocity ratio for a single movable pulley always remains constant (equal to 2) so on lubricating the pulley the frictional forces reduces which increases the mechanical advantage but velocity ratio remains the same and since efficiency is given by

$\text {Efficiency} = \dfrac {\text {Mechanical Advantage}}{\text {Velocity Ratio}}$

so it will also increases.

So, on lubricating the pulley, mechanical advantage and efficiency increases but velocity ratio remains constant.