15 Turning on a pivot 1 Which statement best describes a lever? Tick () the correct box. It changes the size of a force. It changes the direction of a force. 0 It changes the size or direction of a force. It changes the area affected by a force. The turning effect of forces 2 Shazia says that the fulcrum is part of a lever and Ali says the pivot is part of a lever. a) (i) Who is correct? (ii) Explain your answer. Ali asks Shazia what she thinks fulcrum means. b) What should she reply? Shazia says that there are also two other parts to a lever. c) What are they? 3 What is the difference between the load and the effort? The load is The effort is 81TURNING ON A PIVOT CHAPTER 15 Types of levers 6 Who constructed the law of moments? Tick () the correct box. Aristotle effort Democritus load Galileo B Archimedes 7 Here are some parts of the definition of the law of moments but they are in effort load the wrong order. Arrange them in the correct order by writing them out in the space provided. the sum of the clockwise moments about any point effort is in equilibrium the anticlockwise moments about that point load Bodyof to Joetis primat ent equals the sum of when a body Name the class of each lever and give an example. (or balanced) Lever Class Example A B C 8 Shen found a long plank and set it up as a seesaw. His weight is 450 N and he Moments sits 2 metres from the point on which it turns. How far will his younger sister Bo, who weighs 300N, have to sit from the point to balance her brother? Show your working. 5 What is a moment? Tick () the correct box. A moment is a measure of the turning effect produced by a force: around a point supporting the lever. around the ends of the lever. around the effort of the lever. Teacher comments around the load of the lever. 82 8315 Turning on a pivot The turning effect of forces Types of levers Moments The principle of moments At the beginning of Chapter 7, we saw how Ancient Greek philosophers argued about their ideas from what they saw of the world around them. There was one philosopher who was very different. He was called Archimedes (287-212 BCE) and where other philosophers argued he set about finding out about the world by making measurements. The best example of his measurements in science is in his study of the lever. He did not invent the lever - it had been known from the earliest times as a device that can help lift an object. He was the first person to study it scientifically and his work inspired many scientists who came after him - most notably Galileo whose work you have studied in the introductory chapter and Chapter 17 of Student's Book 1. In this chapter, we look at a law Archimedes discovered and examples of the three types of levers. Figure 15.1 Archimedes was so excited about his work on the lever he stated that if he had a long enough lever he could move the Earth. 189PHYSICS 15 TURNING ON A PIVOT a) A first class lever What type of lever is a The turning effect of forces crowbar? 2 In the human arm the elbow effort A force can be used to turn an object in a circular path. For Is the fulcrum, the effort is fulcrum load example, when you push down on a bicycle pedal the cog wheel attached to the crankshaft turns round. produced by the biceps and the A nut holding the hub of a bicycle wheel to the frame is load acts downwards through b) A second class lever effort turned by attaching a spanner to it and exerting a force on the hand. What class of lever is the other end of the spanner in the direction shown in this part of the human arm? Figure 15.2. 3 A bottle opener has its fulcrum fulcrum load A device that changes the direction in which a force at one end of the lever and the acts is called a lever. It is composed of two arms and a effort is applied at the other c) A third class lever effort Figure 15.2 Tightening a nut fulcrum or pivot. The lever also acts as a force multiplier. end. What class of lever is it? This means that a small force applied to one arm of the lever can cause a large force to be exerted by the other ulcrum load arm of the lever. For example, a crowbar is a simple lever. It is used to raise heavy objects. One end of the crowbar is Figure 15.4 Three classes of levers put under a heavy object and the crowbar is rested on the fulcrum (Figure 15.3). When a downward force is applied to the long arm of the crowbar, an upward force is exerted Moments on the heavy object. A small force acting downwards at a large distance from the fulcrum on one side produces The turning effect produced by a force around a fulcrum a large force acting upwards a short distance from the is called the moment of the force. This is best understood fulcrum on the other arm. by referring to the first class lever in Figure 15.4a as you The force applied to the lever to do the work is called read the following text. the effort. It opposes the force that is resisting the The direction of the moment is usually specified as movement, called the load (Figure 15.3). clockwise (the load in Figure 15.4) or anticlockwise (the effort in Figure 15.4) about the fulcrum. The size of the moment is found by multiplying the size of the force by the distance between the point at which the force acts and the fulcrum (in Figure 15.4a these are the distances from the ends to the fulcrum). The moment of a force can be load shown as an equation: moment of force = force x distance from the fulcrum effort The moment is measured in newton metres (Nm). The ulcrum moment of the force applied to one arm of a lever is equal to the moment of the force exerted by the other arm. For Figure 15.3 Using a lever: a simple crowbar example, a 100 N force applied downwards 2 m from the fulcrum produces a 200 N force upwards 1m from the Types of levers fulcrum on the other arm. There are three types of levers, shown in Figure 15.4. They are based on the relationship between the effort, the load and the points of action relative to the fulcrum. 191 190. PHYSICS 4 What is the moment of a 100N In the case of a seesaw, which is another simple type of lever, the moment of the weight on one arm must force acting on a crowbar: equal the moment of the weight on the other arm a) 2m from the fulcrum for the seesaw to balance. From this observation, b) 3m from the fulcrum c) 0.5m from the fulcrum? Archimedes constructed his law of moments, which is 5 A 100 N force acting on a lever sometimes called the law of the lever or the principle of 2m from the fulcrum balances moments. This law or principle states that when a body an object 0.5m from the fulcrum is in equilibrium (or balance) the sum of the clockwise on the other arm. What is the moments about any point (such as the fulcrum) equals the weight of the object (in newtons)? sum of the anticlockwise moments about that point. What is its mass (in kg)? A pair of pliers (Figure 15.5) is made from two levers. 6 Where will the strongest force When they are used to grip something a small force be exerted by scissor blades to applied to the long handles produces a large force at the cut through a piece of material? short jaws. Explain your answer. 7 Why can a lever be described as a force multiplier? Figure 15.5 Using pliers and scissors SUMMARY A force can produce a turning effect (see page 190). A lever is a device that changes the direction in which a force acts (see page 190). There are three types or classes of levers (see page 191). The turning effect produced by a force around a fulcrum is called a moment see page 191). End of chapter question Is it possible to balance a mass of weight 5 N with a mass of weight 15 N on a model seesaw with 10 cm arms? Explain your answer. Suggest where you might place each mass to get an exact balance. 192