What Is a Pulley? Definition, Mechanism & Common Uses Explained
Pulley is a simple metallic or wooden machine which uses a wheel and a rope for lifting heavy loads. Nowadays, for carrying small loads, plastic pulleys are also being used and available in the market. We can rotate it very freely just by the axis passing through its center. It helps in changing the direction of the force which helps in making it easier for the people to lift any object. With the help of a pulley, one can pull down one end of the rope to lift ten Kgs objects which are one meter high.
Types of Pulley
Mainly, there are two types of and those are fixed pulley and movable pulley. These type of pulleys are explained below –
Fixed Pulley – The pulley in which it’s block is fixed on a high platform is known as fixed pulley. An extensible string is passed over the groove where one end of it is attached to the body for being lifted and the other end is free to use. Usually, these types of pulleys can be seen in walls, floors and ceilings. In a fixed pulley, its system is fixed but the rope can move freely.
Movable Pulley – The pulley in which it’s block is not fixed but it carries a load is known as movable pulley. In a movable pulley, an extensible string is tied around the groove where one end of it is fixed to fixed support while the other end of it is always kept free so that the efforts can be applied. As the effort is applied on it, the block which is with the loads moves upward altogether. In the mechanism of a movable pulley, wheels carry the load instead of ropes. In this type of pulley, a little amount of manual work is required.
Compound Pulleys – Compound pulley meaning combines the functions of both fixed and moving pulleys. In this case, the movable pulley links with the rope attached to a fixed one. Accordingly, one can mount the weight on the movable wheel to shift its position. It helps one to redirect force as per one’s requirements, while also altering the overall workload of that force.
Therefore, this sums up the three distinct types of pulleys.
Formula for Pulleys
Mechanical advantage can be calculated by dividing load by effort
Load can be calculated by multiplying mechanical advantage with effort.
Effort can be calculated by dividing load by mechanical advantage.
For finding out the velocity ratio, we have to divide distance of effort by distance of load.
(The ratio of the distance moved by effort and load is the velocity ratio).
Mechanical Advantage of Pulley
Mechanical advantage has been used to measure the effectiveness of the pulleys. It can be calculated when the friction is absent. All the machines have some friction but sometimes somehow if all of the machines overcome friction, the mechanical advantage is somewhat always greater than in the real world.
The mechanical advantage of any pulley is the ratio of the force which is utilized in the work to the force which is applied in the work. During its calculation, we used to make assumptions about the weights of pulley and ropes that they are negligible and there is no loss of energy which occurs due to the friction which is the wheel and ropes. There is one more assumption that the ropes which are used during the operation, do not elongate during the process.
The effort which is required for lifting the weight is equal to the weight of the body in itself in the single loop system.
The effort of pulling the weight becomes half when there are two loops present in the pulley.
FAQs on Pulley in Physics: Types, Formula & Real-Life Applications
1. What is a pulley and how does it fundamentally work?
A pulley is a simple machine consisting of a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt. The wheel's rim is typically grooved. A pulley system works by applying a force to a rope, which then transmits that force to lift a load. Depending on its configuration, a pulley can either change the direction of the force or provide a mechanical advantage, which multiplies the force to make lifting heavy objects easier.
2. What are the main types of pulleys with examples?
There are three primary types of pulleys, each with a distinct function:
Fixed Pulley: The axle is stationary or 'fixed' in one place. It changes the direction of the applied force but does not increase it, providing a mechanical advantage of 1. A common example is the pulley on a flagpole used to hoist a flag.
Movable Pulley: The axle moves along with the load. It multiplies the effort force, providing a mechanical advantage greater than 1, but does not change its direction. These are often used in construction to lift heavy materials.
Compound Pulley (Block and Tackle): This is a system that combines both fixed and movable pulleys. It provides a significant mechanical advantage, making it ideal for lifting extremely heavy loads. Examples include cranes and rigging on sailboats.
3. What is the working principle behind a pulley system?
The working principle of a pulley system is based on distributing force across different sections of a rope. In a system with one or more movable pulleys, the load is supported by multiple strands of the rope. This effectively divides the weight of the load among the strands, reducing the effort force required to lift it. While the force is reduced, the distance the rope must be pulled increases proportionately, adhering to the principle of conservation of energy (where Work = Force × Distance), ignoring frictional losses.
4. How is the Mechanical Advantage (MA) of a pulley system calculated?
Mechanical Advantage (MA) is a measure of how much a machine multiplies the effort force. For a pulley system, the Ideal Mechanical Advantage (IMA) is calculated by counting the number of rope segments that directly support the movable load. For example, if a load is supported by 4 rope segments, the IMA is 4. The Actual Mechanical Advantage (AMA) is the ratio of the load force to the actual effort force required and is always less than the IMA due to energy lost to friction.
5. What is the key difference between a fixed pulley and a movable pulley?
The key differences lie in their structure, function, and mechanical advantage:
Function: A fixed pulley is primarily used to change the direction of force, making it more convenient to pull down to lift an object up. A movable pulley is used to reduce the effort force needed to lift a load.
Mechanical Advantage (MA): A single fixed pulley has an MA of 1 (no force multiplication). A single movable pulley has an MA of 2 (effort is halved).
Structure: The axle of a fixed pulley is stationary. The axle of a movable pulley moves with the load.
6. Where are some common real-world applications of pulleys?
Pulleys are essential simple machines found in many everyday objects and complex systems. Common applications include:
Elevators: Use complex pulley and cable systems to lift the elevator car.
Construction Cranes: Employ block and tackle systems to lift tons of material.
Gym Equipment: Weight machines, like the lat pulldown, use pulleys to redirect force and lift weight stacks.
Window Blinds: A simple pulley mechanism is used to raise and lower the blinds with a cord.
Flagpoles: A fixed pulley allows a person on the ground to hoist a flag to the top.
7. What is the importance of Velocity Ratio (VR) and Efficiency in a pulley?
Both are crucial for understanding a pulley system's performance:
Velocity Ratio (VR): This is the ratio of the distance the effort moves to the distance the load moves. In an ideal pulley system without friction, the VR is equal to the Ideal Mechanical Advantage (IMA). It represents the system's theoretical force-multiplying capability.
Efficiency: This measures how well the pulley converts the work input into useful work output. It is calculated as the ratio of Actual Mechanical Advantage (AMA) to Ideal Mechanical Advantage (IMA), i.e., Efficiency = (AMA / IMA) × 100%. Efficiency is always less than 100% in a real machine because some energy is always lost to friction in the axles and ropes.
8. Does a single fixed pulley actually make it easier to lift an object?
This is a common point of confusion. A single fixed pulley does not reduce the amount of force needed to lift an object; its mechanical advantage is 1. Therefore, to lift a 10 kg load, you still need to apply 10 kg of force (plus extra to overcome friction). However, it makes the task more convenient. It changes the direction of the force, allowing you to pull downwards to lift the object upwards. Pulling down is often easier and safer as you can use your own body weight to assist the effort.
9. Why is a block and tackle system more effective for lifting heavy loads than a single pulley?
A block and tackle system, which is a compound system of multiple pulleys, is more effective because it provides a much higher mechanical advantage. Each time the rope passes from one pulley to another and back to support the load, another segment of rope shares the load's weight. This drastically reduces the effort force required. For example, a system with 5 supporting rope segments reduces the required effort to about one-fifth of the load's weight. While this requires pulling a much longer length of rope, it enables a person to lift loads that would be otherwise impossible to move.
