Fixed Pulley vs Movable Pulley: Detailed comparison

First and foremost, to answer fixed pulley vs movable pulley, we differentiate the pulleys in a few points, and afterward, we will likewise talk about them separately.

Both fixed and movable pulleys are used to lift heavy objects. The rotating axis is the main difference between fixed and movable pulleys. The first has a fixed axis of rotation, and the latter has a movable axis of rotation, as their names suggest.

We’ll start with the fundamental concept of a pulley before going into a fixed pulley vs movable pulley in depth.

The pulley:

A string or rope is wound around the grooved rim of a wooden or metal disc to create the pulley. The disc revolves around its axis, which runs through the centre.

There are two ways to use a single pulley: 

• As a fixed pulley 
• As a movable pulley.

Let’s talk about fixed pulley vs movable pulley, which is the focus of this post.

Fixed pulley vs Movable pulley: Points to remember

The following describes the difference between fixed and movable pulley:

Parameters	A Fixed pulley	A Movable Pulley
Definition	The axis around which the disc rotates is fixed.	The axis around which the disc revolves is movable rather than fixed.
The Load attachment	Here the load one wants to lift is attached to the one end of the rope.	Here the load one wants to lift is attached directly with the pulley.
The direction and magnitude of force	This pulley can only change the direction of the force but can not change the magnitude of the force.	This pulley can change the direction of the force as well as the magnitude of the force.
Use	It is the one which provides better direction to apply effort.	It is the multiplier of force.
M.A.	This pulley’s mechanical advantage is 1.	This pulley’s mechanical advantage is 2.
V.R.	This pulley’s velocity ratio is 1.	This pulley’s velocity ratio is 2.
Example	Pulley used in well	Pulley used in zip lining

Let’s have a look at both pulleys independently now.

A Fixed Pulley:

Fixed pulley refers to a pulley system in which the pulley is mounted to a fixed point and the load is attached to one end of a rope with negligible mass that passes through the grooved rim of the disc or wheel.

Due to load, one side of the rope will be stretched. The stretching creates tension in the rope. The load and the tension created in the rope will have opposite directions. But as the load is the cause of tension, it will have the same magnitude as that of the load applied. If we represent applied load as L and rope tension as T, we can write:

L = T

On the other side of the rope, the person is applying effort to lift the object. Thus, due to effort, again, the rope is stretched, and tension is created. Here the tension is due to effort applied by a person; thus, both have the same magnitude but have opposite directions.

If we represent applied effort as E, we can write:

E = T

Thus, mechanical advantage of fixed pulley is:

∴ M.A. =1

Thus, a fixed pulley does not give you any kind of mechanical advantage. It only provides you a better direction to lift an object with all your weight. It means you need to apply the effort as that of the load weight.

L = E

As the load and effort are the same in magnitude, the distance travelled by both will also be the same. Thus, if we represent distance travelled by the load as L_d and distance travelled by effort as Ed, then we can write:

L_d = E_d

If both distances take the same time to travel, then the velocity ratio is:

∴ V.R. = 1

Thus, efficiency of fixed pulley:

∴ ???? = 100%

Thus the load is lifted upward, same as you apply effort in a downward direction. 

The pulley used in the well, flag pole are examples of the fixed pulley.

A Movable Pulley:

A movable pulley refers to a pulley system in which the pulley is directly attached to the load. One end of a rope with negligible mass that passes through the grooved rim of the disc or wheel is attached to the fixed point, and the other end is free. 

The free end of the rope is used to apply the effort. The load is kept with the pulley between the fixed end and a free end of the rope. Thus, due to the load, there will be tension on both sides of the rope. As load is responsible for the tension in both sides of the rope, we can write:

T + T = L

∴ L = 2T

Now, the applied effort also creates tension in the rope. Thus, we can write:

E = T

∴ L = 2E

Thus, the applied effort will be half of the load.

Thus, mechanical advantage of movable pulley is given by:

∴ M.A = 2

It shows that with lesser effort, more load can be lifted. Thus, this pulley is used as a force multiplying machine.

Now the distance travelled by effort will be doubled from a distance travelled by the load. This happens because by applying effort, not only load but the rope with a fixed point is also lifted. Thus, velocity ratio for movable pulley is:

V.R = 2

Thus, efficiency of a movable pulley is:

???? = 100%

The pulley used in the modern elevator, construction crane are examples of the movable pulley.

The 100% efficiency is only possible in ideal conditions, as friction is always present in reality.  As a result, in reality, M.A. is always < 1, and thus efficiency ???? is also always < 100%.

What does the movable pulley do that a fixed pulley does not? 

A fixed and a movable pulley are both used to lift heavy objects.

A fixed pulley can only provide you a better direction to lift an object. But a movable pulley helps you to lift an object with half of the effort than the load. Thus, using a movable pulley for the same load effort can be reduced, but this can not happen with a fixed pulley.

Also Read:

• Parallel polarization
• Convection vs diffusion
• Amplitude
• Longitudinal vs transverse
• Semiconductors examples
• Gear vs pulley
• Why do deep sea creatures often have bioluminescent parts
• How to find coefficient of static friction
• Newtonian reflectors
• Dew point and altitude