Previously Remember From Page 218 Forces are pushes and pulls that can move or squash objects. An object s speed is the distance it travels every second; if its speed increases, it is accelerating. Unit 2 Forces Why are forces important? We use forces all the time. We push things and pull things. We lift things and throw things. We squash things and break things. To understand all of these actions, we use the idea of forces. In this Unit, you will learn: what forces are, and their effects; how to represent forces; how to measure forces; about important forces including friction, drag, gravity and upthrust; about springs and Hooke s law. Key words air resistance balanced forces contact force drag elastic limit extension force forcemeter friction gravitational field gravity Hooke s law interact lubrication mass newton (N) newtonmeter original length proportional spring unbalanced forces upthrust weight PHYSICS 219
Unit 2 Where do forces come from? If you push or pull something, you are exerting a force on it. A horse can pull a cart. A shark can push its teeth into you. All of these things involve forces. However, it s important to understand that it isn t only living things that can exert forces. If you drop a heavy book on your foot, you will feel the force of the book on your foot. If you sit on a chair, you do not fall on the floor. The chair pushes up on you to stop you falling. Things can exert a force even when they aren t touching. A magnet can attract a piece of steel without touching it. The Sun s gravity pulls on the Earth across 150 million kilometres of empty space. It may seem like magic, but it isn t! Forces appear whenever two objects interact. The Earth s gravity pulls on this astronaut, preventing him from disappearing off into space. How can we measure forces? You can pull a heavy object along the floor. If you pull it using a forcemeter, the meter will show you the size of the force you are using. A forcemeter measures forces in newtons. The newton (N) is the unit of force. Another name for a forcemeter is a newtonmeter. Inside the forcemeter is a spring. The spring stretches as you pull it. The bigger the force, the more the spring stretches. 220 SMART SCIENCE Using a forcemeter to measure the force pulling a piece of wood.
What are contact forces? When two objects touch, each one exerts a contact force on the other. For example, if you lean against the wall, you push on the wall. That is a contact force. At the same time, the wall pushes back on you. The force of the wall stops you from falling over. We can draw these forces as arrows. The arrow shows the direction of the force. The labels tell us the two objects involved. If you put a book on a table, the book pushes down on the table and the table pushes up on the book. force of wall on hand Forces force of hand on wall Draw a diagram with force arrows to show the two contact forces involved when a book is on a table. Label the arrows correctly. What are the effects of friction? When two objects touch each other, another force may appear friction. For example, if you try to push a heavy box along the ground, friction may make it difficult for you. Your push is trying to move the box to the right. Friction acts to the left. Your pushing force must pushing force on box be bigger than the friction force; then the box will move. force of friction on box PHYSICS 221
Unit 2 What causes friction? Friction appears when one surface tries to slide over another. The force of friction acts along the surface. For example, imagine that you are coming down a slide in the playground. You are moving down the slide; friction acts up the slide, slowing you down. Friction is used in the brakes of cars and bicycles. When a cyclist applies the brakes, a rubber pad presses on the rim of the wheel. The force of friction slows the turning of the wheel. friction movement Friction is greater when the two surfaces are rough. Rough surfaces rub against each other. You can reduce friction in two ways. Rub or polish the surfaces to make them smoother. Add a substance such as oil so that the surfaces slide over each other more easily. This is called lubrication. Where is oil used to reduce friction? Give some examples. Is friction always a problem? Friction can be a problem in machines. Oil is used to lubricate machines. Otherwise parts may wear away. Friction can make it difficult to move objects. This can be overcome by putting the object on rollers, for example. However, friction can be useful, too. When we walk, our feet push backwards, and the friction of the ground pushes us forwards. Imagine walking on an icy surface. You can t get a grip because there is little or no friction. We need friction if we are to walk on a smooth surface. backward push of foot on ground forward push of friction on foot 222 SMART SCIENCE
Forces Think about some different sports and the equipment used in them. List some examples where it is important that the equipment is smooth, and other examples where the equipment must be rough. What causes weight? Everything on Earth has weight. Weight is the force that pulls everything downwards. It is the force that makes things fall when you drop them. Weight is caused by the Earth s gravity. Because the Earth has an enormous mass, it pulls on everything in its gravitational field. To escape from the Earth s gravitational field, you would have to go far, far out into space. We can show an object s weight by drawing a force arrow, pointing downwards. The arrow points towards the centre of the Earth. We can measure an object s weight by hanging it from a forcemeter. Remember: weight is a force, so it is measured in newtons (N). Using a forcemeter to measure weight. What is the difference between mass and weight? Don t get confused between mass and weight. An object s mass tells us how much matter it is made of. An object s weight tells us the force of gravity on it. If you travel out into space, your weight will become less. If you go to Mars, your weight will be about one-third of your weight on Earth. If you go to the Moon, your weight will be about one-sixth of your weight on Earth. However, your mass will stay the same. You are still made of the same amount of matter as on Earth. But there is a connection between mass and weight. Your weight is caused by the pull of the Earth. The Earth pulls on every kilogram of mass with a force of about 10 N. So you can calculate your weight by multiplying your mass by 10. weight (in N) = 10 mass (in kg) Calculate the weight of a girl of mass 50 kg. PHYSICS 223
Unit 2 Can liquids and gases exert forces? If you jump into a swimming pool, you will float. Your weight pulls you downwards. The water pushes back upwards. The upward force of water on you is called upthrust (see also page 243). There is upthrust in air, too, but it has a smaller effect than in water. The upthrust of the air makes a hot air balloon rise from the ground. There is another force if you try to move through water or air. If you try to move through water, the force of drag will slow you down. When an aircraft flies through the air, the force of air resistance slows it down. Drag and air resistance are a bit like friction, but they happen in fluids (liquids and gases). A shuttlecock used in badminton is designed so that there is a lot of drag as it moves through the air. Otherwise it would fall too quickly. A shark has a streamlined shape so that it can move quickly through water and catch its prey. shuttlecock upthrust of water weight of swimmer shark Draw the outlines of two cars, one more streamlined than the other. Think about what you have experienced in a swimming pool. Describe what it is like if you try to wade through the water. Explain why it is difficult to reach the bottom at the deep end. 224 SMART SCIENCE
How do forces change the way things move? To work out how forces will make an object move, we must only think about the forces acting on the object. In these examples, a longer arrow means a bigger force. The forces on The pushing force is The pushing force is greater this object cancel out. These are balanced forces. The object does not move. upthrust cancelled out by the force of friction. Again, the forces are balanced. The object does not move. than the force of friction. These are unbalanced forces. The object starts moving to the right. Forces friction push friction push weight Only one force acts on the ball. It falls downwards. It gets faster as it falls it accelerates. The drag of the air pulls upwards on the parachutist. The drag force is greater than his weight and so he falls more and more slowly he decelerates. air resistance weight weight If the forces on an object are balanced, it will remain still (or carry on moving at a steady speed). If the forces are unbalanced, it will start moving, speed up or slow down. 1 Just before he lands, are the forces on the parachutist balanced or unbalanced? Explain your answer. 2 A cyclist brakes in order to stop at a red light. While she is slowing down, are the forces on the bicycle balanced or unbalanced? Explain your answer. PHYSICS 225
Unit 2 What happens when forces stretch a spring? Forces can make a spring get longer. If you hang a spring from a clamp and add weights to the bottom of it, the spring will stretch. There are two forces at work on the spring. The clamp is pulling upwards. The weights are pulling downwards. The bigger the force applied to the spring, the more the spring stretches. If you remove the weights, the spring will go back to its original length. Take care! Don t apply too much force to a spring or it will break. original length stretched length extension weights In 1660, Robert Hooke investigated springs. He measured the length of a spring as he added weights to it. You can investigate his ideas. The pictures show what needs to be measured. Measure the original length of the spring, when no weights are pulling on it. Measure its length when weights have been added. Now calculate the increase in length, which we call the extension: extension = stretched length original length A spring is 25 cm long when it is unstretched. A force of 100 N stretches it to a length of 32 cm. Calculate the extension produced by the force. Some springs are designed to be compressed (squashed). The springs in a car s suspension compress as the wheels roll over bumps in the road. This helps to keep the car level and smoothes out the ride for the passengers. Foam rubber can be stretched or compressed. Explain how you could investigate the effects of forces on a block of foam rubber. 226 SMART SCIENCE
What is Hooke s law? Hooke discovered that if the force on a spring is increased in equal steps, the extension also increases in equal steps. We can show this as a graph. The force is on the y-axis. The extension is on the x-axis. The graph for small forces is a straight line through the origin (zero). This shows that the extension is proportional to the force. The graph for large forces bends sharply to the right. This shows what happens when the spring has been stretched by a force that is too big. The spring will not return to its original length. The point where this happens is called the elastic limit. The graph shows Hooke s law. The extension of a spring is proportional to the force producing it, providing that the elastic limit is not exceeded. force (N) elastic limit Forces extension (cm) You need to remember that: Forces appear when two objects interact with each other. We measure forces using a newtonmeter (a forcemeter). The unit of force is the newton (N). We represent forces using force arrows. The force of friction can arise when two objects touch each other. Friction is the force that enables us to walk. We can reduce unwanted friction by using lubrication. The weight of an object is a force caused by the pull of the Earth s gravity. On the Earth s surface: weight (in N) = 10 mass (in kg) q q The force pushing upwards on objects in water is called upthrust. When objects move through liquids or gases, they experience drag. When the forces on an object are balanced, its motion does not change. Unbalanced forces change the motion of a body it speeds up or slows down. Hooke s law states that the extension of a spring is proportional to the force producing it, providing that the elastic limit is not exceeded. Next time Unit 3 Forces can be made to turn things using levers (page 228). Unit 5 Pressure tells us about how a force is spread out over an area (page 241). PHYSICS 227