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Block and Tackle System - Important Topic for JEE

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Last updated date: 20th May 2024
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What is a Block and Tackle System?

A block and tackle, which is also frequently known as tackle, is a system of two or more pulleys. A rope, cable, or thread is threaded between the pulleys, and this system is used to lift heavy loads and reduce the effort required. You've probably seen a block and tackle at work if you've ever glanced at the end of a crane, or if you've ever used an engine hoist or a come-along, or if you've ever looked at the rigging on a sailboat. A block and tackle is a rope and pulley system that lets you swap force for distance.


The pulleys in a block and tackle are assembled to form blocks. In a simple two pulley block and tackle, these blocks are paired such that one pulley is fixed, and the other pulley is allowed to move with the load at the end of the rope. The rope is threaded through the pulleys to create a mechanical advantage, which increases the force exerted on the rope. One common example of the block and tackle system is the elevator.


Block and tackle system.


Block and tackle system


What is a Pulley?

A pulley is a wheel on an axle or shaft that supports movement and direction changing of a taut cable or belt, as well as power transfer between the shaft and the cable or belt. The supporting shell is termed a block, and the pulley is called a sheave in the case of a pulley supported by a frame or shell that does not deliver power to a shaft but is used to guide the cable or exert a force. 


To locate the cable or belt, a pulley may include a groove or grooves between flanges around its circumference. A rope, cable, belt, or chain can be used to drive a pulley system. Below is the diagram of a simple pulley.


A pulley


A pulley


Rope and Pulley Systems

The use of a single continuous rope to convey a tension force around one or more pulleys to lift or move a load is characterised by a rope and pulley system, also known as a block and tackle. The rope can be a light line or a robust cable. The list of simple devices identified by Renaissance scientists includes this system.


The forces acting on the moving block must add up to zero in order for the system to be in equilibrium. Furthermore, the tension in the rope must be consistent across all of its components. This means that each half of the load must be supported by the two halves of the rope supporting the moving block.


The rope and pulley systems can be easily solved using Newton’s second law of motion. The respective forces on the pulley, the blocks, and the ropes need to be equated, and then the equation of motion can be solved. 


Types of Pulley

Different types of pulleys are used in a block and tackle system. The different types of pulleys have their own advantages, and they are used in different types of applications. The pulley systems can be broadly divided into three types. These are listed below:

  1. Fixed Pulley

An axle is positioned in bearings attached to a supporting structure on a stationary pulley. The force on a rope or belt that moves around its circumference is changed by a stationary pulley. Combining a fixed pulley with a movable pulley or another fixed pulley of a different diameter provides a mechanical advantage.


  1. Movable Pulleys

As the name suggests, a movable pulley is a pulley that can move. It is attached to a movable block via an axle. A single movable pulley has a mechanical advantage of two since it is supported by two portions of the same rope.


  1. Compound Pulleys

Compound pulleys can be termed as the system of pulleys that if formed from a combination of fixed and movable pulleys. As we already saw in the definition of a block and tackle system, we can say that compound pulleys are basically block and tackle systems. The mechanical advantage of a block and tackle can be increased by mounting many pulleys on the stationary and movable axles.


Mechanical Advantage of Block and Tackle System

The most basic difference between a block and tackle system and a pulley is that a block and tackle system is a system of pulleys. A single pulley is just a wheel on an axle with some rope or cable running over it. Pulleys are useful machines, but they don’t reduce effort. They just change the direction of any force applied to an object, like when one pulls water out of a well. 


On the other hand, a system of pulleys, or more precisely, a block and tackle system can multiply force. Since there are pulleys, they also change the direction of forces. Since block and tackle systems can multiply force, they are advantageous over pulleys because they can reduce effort. 


The mechanical advantage of the block and tackle is the mismatch between the amount of force required to move an object and the weight of the object. This is similar to the difference between the amount of rope we pull and the distance the thing travels. 


Example of the Advantage of a Block and Tackle System

Things will become clear with an example.


Suppose we have a crate that weighs 10 N, and we need to pull it up from the floor to the roof. If we put a pulley on the roof and then put a rope through it, which is tied to the crate, we would have a simple pulley system. This pulley system will change the direction of the force, but it will not reduce effort. If we pull the rope up by one metre, the crate will also travel one metre. This means that we have to apply a force of more than 10 N to pull the crate. 


Now, if instead of attaching the rope directly to the crate, we passed it through a new pulley that is attached to the crate, we would get a block and tackle system. When we pull the crate now, the rope has to travel through two pulleys. This means that if we pull the rope by 2 m, the crate will only move up by 1 m. However, in this case, we wouldn't exactly be pulling the whole weight of the crate. We would need to apply a force of more than just 5 N instead of 10 N to pull the crate. This way, our effort is reduced.


Two pulley block and tackle system.

Two pulley block and tackle system


The mechanical advantage of a block and tackle machine is determined by the number of rope lengths between the two pulleys. The rope in our machine runs from the top pulley to the lowest pulley and back up to the rafters, giving us a mechanical advantage of two.


Conclusion

A block and tackle system is just a system of two or more pulleys that helps to reduce the effort of pulling anything. A pulley is a wheel on an axle or shaft that allows a taut cable or belt to move and change directions while also transferring power between the shaft and the cable or belt. There are three types of pulleys: fixed pulleys, movable pulleys, and compound pulleys. Compound pulleys are a block and tackle system. While pulleys change the direction of force applied to any object, block and tackle systems also reduce the effort in addition to changing the direction of the forces. This is the mechanical advantage of the block and tackle system. 

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FAQs on Block and Tackle System - Important Topic for JEE

1. How do you keep a block and tackle from twisting?

Blocks are often twisted when they are being worked with. It is known as cabling and is a big problem. When this happens, the wire rope is frequently blamed, while other equally critical aspects of the process are ignored. The twisting can be avoided by reducing the wire rope length, reducing the amount of load being lifted, or by restraining the twisting block with a tagline. You can also increase the sheave size, relocate the rope dead end at the bottom, and even eliminate odd parts of the reeving. 

2. What are the real-life uses of pulleys?

A pulley is a simple machine that has variable uses in our everyday lives. They are used in many exercise equipment, curtains at a theatre are also moved by using pulley systems, and blinds on windows also operate using pulley systems which help them to move up and down. Flagpoles also use pulleys to hoist flags up or bring them down when needed. Another use of a pulley is in a crane, and even a bulldozer uses a pulley system. There are many similar examples.