Question

A movable pulley is used as:
a) force multiplier
b) speed multiplier
c) device to change the direction of effort
d) all the above

Answer 1

A movable pulley is attached with the string and a fixed pulley; it can move when load is attached to it. After attaining equilibrium, the whole system attains a stable state and after balancing the force, we will get a relation between force and load.

Complete answer:
Consider a movable pulley P1 and a fixed pulley P2 used to lift up load W.

P2 is fixed pulley, tension along pulley P2 is equal to the E.
Total upward tension in pulley $P1 = 2T$
Total downward force in pulley $P1 = W$
At equilibrium, net force is zero.
$\textit{upward force} = \textit{downward force}$
$2T = W$
$T = \dfrac{W}{2}$
The movable pulley decreases the force required to pull a load by a multiple of $2 \times$ movable pulleys number. The force will be half of that weight lifted in a movable pulley as the massless pulley forces should balance. So, a Movable pulley is used as a force multiplier.

Option (a) is correct.

Additional Information:
A movable pulley has additional numbers of strings used to raise mechanical advantage, and that advantage is load can elevate by implementing a force equivalent to half the load. A movable pulley is supported by two parts of the exact string and produces mechanical support.

Note:
The mechanical advantage is the fraction of the load to the tension of the system. The mechanical advantage of a pulley system balances the rope's tension force with the force of gravity on the load. The frictionless and massless pulleys do not disperse energy and change the direction of a rope that does not stretch. A force balance on the body of load W and n supporting sections of a rope with tension T gives:
$nT – W = 0$
$n = \dfrac{W}{T}$
Thus, the system's mechanical advantage is equal to the number of rope sections carrying the load.