4.9 A pulley changes the size or direction of a force

4.9

A pulley changes the size or direction of a force

Learning intentions

By the end of this topic, you will be able to: • explain how a pulley makes it easier to lift a load • calculate effort in a pulley system.

Figure 1 Sailing ships have pulley systems for lifting heavy sails and cargo.

Watch it! A video and quiz on pulleys

Key ideas

• A pulley is a simple machine that makes it easier to lift an object. • Pulleys are wheels with a groove along their edge. • The wheel is used to change the size or direction of the force used.

History of the pulley

Between the 15th and 17th centuries, a period known as the Age of Discovery, the people of Europe were desperate for spices, gold and silver. Sailors navigated the seas looking for these treasures. They returned with large amounts of bounty that included food, weapons and slaves. All this cargo needed to be loaded on and off the ships as quickly as possible. To help the sailors do this work, they used a simple machine invented by Archimedes many centuries before. Types of pulley The simplest pulley system is made of one pulley. This system only changes the direction of the applied force, not the size of the force. As a person pulls down on the rope, the weight on the other end moves up (Figure 2). This does not change the amount of effort needed, but it makes lifting easier. This is because the person can use their weight to help in the lifting. You have probably used this type of pulley when you pull the cord to open a window blind at home. (Figure 3) The mechanical advantage is calculated by the number of ropes between the upper and lower pulleys; in the roller blind system, it is one.

The more pulleys that are used, the easier it is to lift a load because its mechanical advantage is increased. For example, if two pulleys are used, the system can lift twice the load of a single-pulley system. The mechanical advantage of this system is 2 (Figure 4). Load Figure 2 A single-pulley system – rope is guided through the groove of a rotating wheel. DRAFT

Figure 3 The simplest pulley system is made of one pulley. Load

Figure 4 A two-pulley system doubles the mechanical advantage.

Worked example 4.9 Calculating effort

Calculate the effort needed to lift a load of 150 newtons with a frictionless block and tackle with fi ve pulleys.

Solution Mechanical advantage = 5 (5 pulleys)

Load Load = 150 newtons Mechanical advantage =

size of the load size of the effort

5 = 150 N

effort

Comprehend 1 Explain why two pulleys are better than one. 2 Describe three examples where single pulleys or pulley systems are used. 3 Describe how pulleys have made loading and sailing huge cargo vessels possible. Analyse 4 A block and tackle provides a mechanical advantage because it can lift heavy loads.

Infer the disadvantages of this system. 5 Identify the correct option. A pulley system can: a increase force and distance at the same time

Figure 5 A four-pulley system increases the mechanical advantage by a factor of four. A four-pulley system can magnify the effect of the effort four times. For example, a 25 kg effort can lift a 100 kg load in a frictionless pulley system. The simple system shown in Figure 5 not only changes the direction of the applied force, but it also multiplies it by four. This system has a mechanical advantage of 4. Worked example 4.9 shows how to calculate effort in a pulley system. Groups of pulleys are often mounted together in a frame or ‘block’. This device is called a block and tackle . A small effort pulling through a long distance lifts a large load through a much smaller distance. 4.9 Check your learning

b decrease distance while increasing force c decrease force and distance at the same time. 6 A 100 kg mass is used to lift an 800 kg mass. a Calculate how many pulleys are needed to make this possible. b Calculate the mechanical advantage of this machine.

Effort = 150 N 5

= 30 N The effort needed is 30 newtons. Figure 6 A block and tackle DRAFT Quiz me Complete the Quiz me to check how well you’ve mastered the learning intentions and to be assigned a worksheet at your level. block and tackle a group of pulleys mounted together in a frame or block, which provides signifi cant mechanical advantage