Contents Contents Stage 7 1 1.1 Introduction to forces 8 1.2 Balanced forces 10 1.3 Friction 12 1.4 Gravity 14 1.5 Enquiry: Questions, evidence and explanations 16 1.6 Air resistance 18 1.7 Enquiry: Planning investigations 20 1.8 Tension and up 22 1.9 Enquiry: Presenting results tables and line graphs 24 1.10 Extension: A journey into space 26 1.11 Review: Checkpoint-style questions 28 2 Energy 2.1 What is energy? 30 2.2 Energy from the Sun 32 2.3 Energy types 34 2.4 Energy transfer 36 2.5 Conservation of energy 38 2.6 Storing energy potential energy and GPE 40 2.7 Storing energy elastic potential energy 42 2.8 Enquiry: Suggesting ideas and collecting evidence 44 2.9 Extension: Energy calculations and Sankey diagrams 48 2.10 Review: Checkpoint-style questions 50 3 The Earth and beyond 3.1 How do we see the Sun, stars and planets? 52 3.2 Day and night 54 3.3 What causes seasons? 56 3.4 Why do the stars seem to move? 58 3.5 Our solar system 60 3.6 Extension: The Moon 62 3.7 Enquiry: Questions, evidence and explanations 64 3.8 Extension: Beyond our Solar System 70 3.9 Enquiry: Using secondary sources 72 3.11 Extension: The Origin of the Universe 74 3.12 Review: Checkpoint-style questions 76 Stage 7 Review 78 Stage 8 4 4.1 Speed 80 4.2 Enquiry: Taking accurate measurements 82 4.3 Distance-time graphs (including interpretting data from secondary sources) 84 4.4 Extension: Acceleration and speed-time graphs 86 4.6 Enquiry: Presenting results in tables and graphs 88 4.7 Enquiry: Asking scientific questions 90 4.8 Review: Checkpoint-style questions 92 5 Sound 5.1 Sound, vibrations and energy transfer 94 5.2 Extension: Detecting sounds 96 5.3 Loudness and the decibel scale 98 5.4 Loudness, amplitude and oscilloscopes 100 5.5 Pitch and frequency 102 5.6 Enquiry: Making simple calculations 104 5.7 Extension: Echoes 106 5.8 Review: Checkpoint-style questions 108 6 Light 6.1 What is light? 110 6.2 How do we see things? 112 6.3 Extension: The speed of light 114 6.4 Reflection at plane surfaces 116 6.5 The law of reflection 118 6.6 Refraction at boundary between air and water 120 6.7 Refraction at boundary between air and glass 122 6.8 Dispersion of white light 124 6.9 Colour 126 6.10 More on colour 128 6.11 Enquiry: Asking scientific questions 130 6.12 Extension: Lasers 132 6.13 Review: Checkpoint-style questions 134 7 Magnetism 7.1 The properties of magnets 136 7.2 The magnetic field of a bar magnet 138 7.3 Electromagnets 140 7.4 Enquiry: Identifying and controlling variables 142 7.5 Extension: Using electromagnets 144 7.6 Review: Checkpoint-style questions 147 Stage 9 8 8.1 Pressure, and how it is caused 150 8.2 Pressure and its applications 152 8.3 Pressure in liquids 154 8.4 Using pressure in liquids 156 8.5 Pressure in gases 158 8.6 Extension: Pressure and temperature in gases 160 8.7 Enquiry: Preliminary work 162 8.8 Density and how to measure it 164 8.9 Explaining density 166 8.10 Enquiry: Questions, evidence and explanations 168 8.11 Turning forces 170 8.12 Enquiry: Planning investigations 172 8.13 Calculating moments 174 8.14 Extension: Centripetal forces 176 8.15 Review: Checkpoint-style questions 178 9 Electricity 9.1 Electrostatic phenomena 180 9.2 Explaining electrostatics 182 9.3 Digital sensors 184 9.4 Electric circuits what can you remember? 186 9.5 Current what is it and how can we measure it? 188 9.6 Parallel circuits 190 9.7 Explaining parallel circuits 192 9.8 How components affect current 194 9.9 Voltage 196 9.10 Enquiry: Choosing ideas and making plans 198 9.11 Extension: Electrical energy and power 200 9.12 Review: Checkpoint-style questions 202 10 Energy 10.1 Hot and cold 204 10.2 Energy transfer conduction 206 10.3 Energy transfer convection 208 10.4 Energy transfer radiation 210 10.5 Cooling by evaporation 212 10.6 The world s energy needs 214 10.7 Fossil fuels 216 10.8 Generating electricity introduction 218 10.9 Renewable energy sources solar and geothermal 220 10.10 Renewable energy sources wind, wave, tidal, hydroelectricity 222 10.11 Energy for the future 224 10.12 Review: Checkpoint-style questions 226 Stage 9 Review 228 Reference pages 230 Glossary 240 Answer 246 Index 250 Key: Enquiry / Extension / Review BOLD PAGE NO. = in this booklet A dashed line to the left of the text indicates extension material. Stage 8 Review 148 1
1.1 Describe different types of force Describe the effects of forces on moving objects Describe how to measure forces Introduction to forces The effect of forces When an object slides across a surface it will experience a force of friction. It is friction that will try to stop something moving. Air resistance is a force that acts on any object moving through the air, and an object moving through the water will experience water resistance. Both air resistance and water resistance are types of drag. The object collides with the particles in the air or the water and that is why it slows it down. A floating object experiences up because the water is pushing up. Balloons also experience up because the air below the balloon pushes up. When you pull something with a rope there is a tension in the rope. In cars, aircraft and other things with engines there is a that pushes the car forwards. You cannot see forces but we can see what they do. Forces are pushes or pulls that can change the shape of objects, make things start to move, or stop them if they are moving. Forces can speed things up, slow them down or change the direction of motion. Forces arrows You can show the forces acting on an object by drawing arrows. The length of the arrow shows the size of the force. The direction of the arrow shows the direction of the force. Forces act on objects, so the arrow always starts on the object. Measuring forces You can use a force meter, such as a spring balance, to measure the size of a force. The spring inside a spring balance is elastic, which means that it goes back to its original length when the force is removed. This is one of the reasons that force meters contain springs. Another reason for using a spring is that it will stretch evenly. Forces are measured in newtons (N). Different types of force The gravitational force, or gravity, is the force that attracts you to the Earth, and it is also the force that attracts the Earth to you! The force of gravity acts between any objects that have mass. On Earth, the force of gravity on an object is called its and acts towards the centre of the Earth. This explains why you are always pulled down, wherever you are on the Earth. There is an electrostatic force between objects that are charged. When objects are charged with static electricity they can attract or repel other charged objects. Magnets attract or repel other magnets. Magnets attract magnetic materials like iron, steel or nickel. There is a magnetic force between them. Like gravitational forces and electrostatic forces, objects do not need to be in contact to experience the force. 1 Copy and complete the table. Tick the column that you think applies to each situation: The force of gravity acting on a falling apple The force of gravity acting on a basketball thrown towards the basket A car braking in a straight line Changes the speed of the object Changes both the speed and direction 2 Why don t people on the other side of the world fall off? 3 Why are springs often used to measure forces? 4 A force can be a push or a pull. Which of the types of force described above can be both? 5 Draw a picture of a boat with a motor. Draw arrows to show, air resistance, up and and label the arrows. Weight, electrostatic force, magnetic force, friction, air resistance, water resistance, up, tension and are all types of force. Forces can change the direction of a moving object, or make it speed up or slow down. Forces are measured in Newtons with a spring balance. 2 3
1.2 Balanced forces The force of gravity, or, is acting on the diver in all the pictures below. Lots of forces Sometimes more than one pair of forces is acting on an object. The of the engine and the air and water resistance are balanced. The boat moves forwards with a steady speed. Explain the difference between balanced and unbalanced forces The and the up are balanced. The boat does not move up or down. up Describe the effect of balanced forces Describe the effect of unbalanced forces water and air resistance Resultant forces Why is the diver moving in only one of the pictures? Balanced and unbalanced forces Two forces can be added together to find the resultant. If the arrows are in the same direction the forces are added together. If the arrows are in opposite directions one force is taken away from the other. air resistance If the forces on an object are the same size but in opposite directions they cancel each other out. They are balanced. The object behaves as if there is no force acting on it in either direction. 6N 10N If the forces acting on an object are balanced then its motion will not change: it will stay still it will keep moving at a steady speed. If the forces on an object do not cancel each other out then they are unbalanced. If the forces are unbalanced then the motion of the object will change: - The resultant force is 4N to the right. - The resultant force is 8N to the left. - The resultant force is zero. If the resultant force is zero the forces are balanced. If the resultant force is not zero the forces are unbalanced. - The of the leaf and the air resistance are balanced. The leaf falls with a steady speed. it will start moving (if it wasn t moving before) it will accelerate (speed up) or decelerate (slow down) it will change the direction it is moving in. friction and air resistance 1 A boy pushes a toy car along the floor at a steady speed. Sketch a picture of the car and draw arrows to show the forces acting on it. Label the forces. 2 Complete the final two columns in the table. Look at the size of the forces and decide if the forces are balanced or unbalanced. The car is currently moving with a steady speed. What will it do? Thrust of car Friction Air resistance Balanced or unbalanced forces? 1500N 700N 800N Speed up/ slow down/ steady speed Forces are balanced if they cancel each other out and unbalanced if they do not. Unbalanced forces change the speed or direction of motion. Objects stay still or move at a steady speed if the forces on it are balanced. 3000N 500N 100N 1000N 600N 500N 2500N 900N 400N - The forces acting on a cyclist. 3 Extension: Vil and Alom are arguing about the forces on a car. Vil says that if the car is moving there must be a resultant force acting on it. Alom says there doesn t need to be a resultant force acting on it. Who do you agree with, and why? 4 5
1.6 Explain what affects air resistance Explain how and why parachutes are used to slow down cars or skydivers Air resistance What is air resistance? When any object moves through air there is a force on it called air resistance. It is easy to demonstrate the effect of air resistance. Take two pieces of paper and screw one piece up into a ball. If you drop the ball of paper and the sheet of paper, the ball reaches the ground first because there is less air resistance. Any object moving through the air will collide with the particles in the air. It is the collisions with these particles that provide the resistance. The sheet of paper has a bigger area in contact with the air, so there are more collisions with air particles. That means that it experiences more air resistance than the ball of paper. Air resistance is also affected by the speed of the object moving through the air. Objects moving with a higher speed will experience more air resistance. When an aeroplane lands the pilot uses the flaps on the wings to increase the air resistance. This reduces the speed so that he can take the aeroplane from the runway to the airport building. The flaps on the wings of an aeroplane help to slow it down after it has landed. Reducing air resistance Air resistance can be a problem. It is a form of friction that slows things down. Air resistance is reduced when the area in contact with the air is reduced. Streamlining reduces air resistance changing the flow of air over a car or plane. Scientists use wind tunnels to experiment with the shape of vehicles and find the best shape. Using air resistance Air resistance can be very useful for slowing things down. A parachute increases the area that is in contact with the air, and therefore increases the air resistance. When a parachute is opened by a skydiver the air resistance increases and so the speed of the skydiver is reduced. The skydiver will slow down enough to land safely. Cyclists pull in their arms and crouch forward to reduce the area in contact with the air. They make themselves more streamlined by using special helmets. 1 Draw a diagram showing the forces acting on the piece of paper being dropped on Earth and on the Moon. 2 Explain in terms of air particles why air resistance depends on the speed of an object. 3 Copy and complete these sentences about the forces on a skydiver jumping out of a plane using the words balanced or unbalanced. Air resistance depends on the speed of the object and the area in contact with the air. A parachute increases air resistance and reduces the speed of an object. When the skydiver jumps out of the plane the forces are. When the skydiver reaches a speed of about 200km/hour the forces are. The parachute opens and this makes the forces. The skydiver has landed and is standing on the ground. The forces are. _ Rocket cars use parachutes for braking. 6 7
Enquiry 1.7 Understand how to plan an investigation to test an idea in science Planning investigations Streamlining Kasini was watching a film about dolphins. Dolphins have to swim fast to catch fish. She wondered what factors affect how fast things can move through water. Suggesting ideas to test Kasini decided to drop objects into a cylinder of water and see how long they took to hit the bottom. These are some of the ideas that she thought about testing: How does the shape affect the time to hit the bottom? How does the mass affect the time to hit the bottom? Making a plan and choosing equipment This is what Kasini wrote down for her plan: I am going to investigate how long it takes different shapes of clay to reach the bottom of the cylinder of water. I will make different shapes from the same amount of clay. These are the shapes I have chosen: cone shape, cube, sphere, cylinder, rectangular I will time how long it takes for the shape to hit the bottom with a stopwatch. This is a list of my equipment: * A large cylinder * Modelling clay * A stopwatch * A balance * A measuring jug I will put my results in a table. Investigations are ways of obtaining evidence to answer a question. Before Kasini could write down a plan for her investigation she had to decide exactly which question she wanted to answer. This is what she wrote: I have decided to investigate how the shape of an object affects how long it takes to fall through water In an investigation the things that can be changed are called variables. Here is a list of the variables that Kasini thought of for her investigation: the shape of the object the mass of the object the volume of water in the cylinder the temperature of the water. The one variable that she decided to change was the shape of the object. It is very important to change only one variable at a time. She decided that she would use the same mass of clay each time and the same volume of water at the same temperature each time. Making improvements Kasini completed her investigation and wrote down her results. She discussed her investigation with her friend Nadia. Nadia asked if there were any problems with the investigation. Kasini said that it was difficult to see when to start and stop the stopwatch because all the shapes moved through the water quickly. Shape Time Cone 0.58 Cube 0.65 Sphere 0.61 Cylinder 0.75 Rectangle 0.68 Making a prediction In school Kasini had learned that engineers design cars and airplanes to be as streamlined as possible to reduce drag. She knew that air resistance was a form of drag, and so was water resistance. She used this information to make a prediction. 1 Copy and complete this table to explain why Kasini needs each of the pieces of equipment. Equipment A large cylinder Modelling clay A stopwatch A balance A measuring jug Why Kasini needs it 2 Why is it important to change only one variable at a time? 3 Sometimes it is hard to see when to start and stop the stopwatch. How can the plan be improved to get better results? 4 What has Kasini missed out of her results table? 5 What type of graph will Kasini be able to plot with her results? An idea can be tested by carrying out an investigation. Variables are things that can be controlled, changed or observed. Scientific knowledge can be used to make predictions. I predict that a cone shape will reach the bottom in the shortest time. This is because there will be less water resistance as a cone is a streamlined shape. 8 9
Extension 1.10 Describe how rockets take off Know Newton s Laws of Motion Apply ideas about forces to objects returning to Earth A journey into space On 29th June 2012 the first Chinese woman to go into space, Liu Jang, returned safely to Earth in the Shenzhou-9 space craft, called a capsule. She had spent 13 days in space together with two other astronauts in Tiangong 1, the first Chinese space station. The space station contains an exercise station, work area and two beds and the astronauts say that it is very comfortable. Getting to the space station It takes a large force to move three people from the surface of the Earth to a space station. The astronauts sit inside a capsule which is attached to a rocket. The fuel inside the rocket is burnt to produce hot gases. These gases rush out the back of the rocket and this pushes the rocket upwards. This is an example of one of Newton s Laws of Motion. Newton s Third Law says that forces come in pairs. The force of the rocket on the hot gases is equal to the force of the gas on the rocket. The force of the gas on the rocket is big enough to make the rocket accelerate away from the Earth towards the space station. Living in space While astronauts work on a space station, like the Chinese Tiangong 1 space station, the Russian Mir space station or the International Space Station, they are orbiting the Earth at a steady speed. The force of gravity is acting on the space station all the time, but the space station does not accelerate or decelerate. The force of gravity is changing the direction of motion of the space station keeping it in orbit. If you could turn off the Earth s gravitational field then the space station would travel in a straight line. This is another of Newton s Laws. Newton s First Law says that an object will travel in a straight line at a steady speed unless a resultant force acts on it. Getting back to Earth The force of gravity will bring the capsule back to Earth. As the capsule re-enters the atmosphere there will a large amount of friction, or air resistance, due to the collision of the air particles with the outside of the capsule. This is a good thing because it slows the capsule down. The friction heats up the outside of the capsule which can reach a temperature of over 1000 C. The scientists have to design the capsule so that the astronauts are safe. When the capsule is about ten kilometres above the ground the parachute is released and the capsule will reach terminal velocity. Finally, when the capsule is about one metre off the ground very small rockets are used to slow it down so that it is safe to land. The journey into space and back contains lots of examples of Newton s Second Law, which says that a resultant force will change the speed or direction of motion of an object. How quickly the speed changes (acceleration) depends on the size of the force and the mass of the object. It takes a bigger force to produce the same acceleration of a bigger mass. A bigger force produces a bigger acceleration if the mass is the same. 1 When a rocket is taking off there are two forces acting on it, its and the force of the air on the rocket. The of the rocket, including the fuel, is 5 million newtons. a Is the force of the air on the rocket bigger, smaller or equal to 5 million newtons as it takes off? b Eventually the fuel will run out. Is the now bigger, smaller or equal to 5 million newtons? c How many forces are acting on the rocket now when the fuel has run out? d What will happen to the speed of the rocket when the fuel has run out? 2 Copy this diagram of the capsule when it is about to land. Add arrows to show the forces on the capsule when the small rocket motors are fired. Label the arrows. 3 In ten seconds a car will reach a bigger speed (accelerate more) than a cyclist. This is an example of which of Newton s Laws? 4 Steep mountain roads often have run-off tracks in case cars lose control. This is an example of which of Newton s Laws? 5 An octopus or a squid pushes out water backwards so that they move forwards. This is an example of which of Newton s Laws? Rockets are pushed up when hot gases are pushed downwards. Newton s Laws describe the links between forces and how objects move. { Newton s First Law: an object will stay at rest or travel in a straight line at a steady speed unless an external force acts upon it. { Newton s Second Law: the acceleration of an object is dependent on size of the force and the mass of the object. { Newton s Third Law: forces come in pairs when two objects exert forces on each other they will produce reaction forces that are equal in size but acting in opposite directions. 10 11
1 What is the difference between balanced and unbalanced forces? Copy out the table below and put a tick in the correct column for each statement: The object is not moving There is only one force acting on the object The object is accelerating The two forces are the same size but in opposite directions The object is slowing down The two forces are different sizes but in opposite directions The object is moving with a steady speed Forces are balanced Forces are unbalanced 2 In the diagrams below the forces acting on the objects are shown by arrows. A. Review 1.11 friction and air resistance [5] a In which diagram are the forces incorrectly labelled? [1] b In which diagram are all the forces balanced? [1] c In which diagram is the moving object decelerating? [1] 3 The Tour de France bicycle race takes place every year. It is a road race and in some places the road is flat, but sometimes racers cycle uphill and then downhill. All of the sentences below are wrong. Change one word in each sentence to make it correct. a When the racer is decelerating uphill the forces acting on him are balanced. [1] b Air resistance acts in the direction of motion of the cyclist. [1] c When friction and up are equal to the racer is accelerating. [1] 4 A streamlined car uses less fuel than if it was not streamlined. a What effect does streamlining have on the air resistance acting on a car? Explain why this happens. [2] b Why does this reduce the amount of fuel that the car needs? Use the words force and in your answer. [2] Extension: up B. C. friction and air resistance up 1 a Newton s Second Law says that the acceleration of an object depends on the resultant force and the mass of the object. Explain what is meant by a resultant force. [1] b Put these vehicles in order of the size of force, from small to large, required to produce the acceleration. D. tension Mass of vehicle Top speed Time to reach top speed A Large 50km/s 20 seconds B Small 50km/s 20 seconds C Small 100km/s 10 seconds D Small 100km/s 20 seconds [4] 12