Welcome to your Complete Physics for Cambridge Secondary 1 Student Book. This book has been written to help you study Physics at all three stages of Cambridge Secondary 1.
Most of the pages in this book work like this:
Introduction to forces
What is a force?
When a tennis ball hits the ground, a force changes its shape, speed, and direction. A force is a push or pull that can change the shape of an object, or change the way that it moves.
You cannot see forces but you can see what they do. If something starts to move, or speeds up, a force is acting on it. Forces can also slow things down or stop them moving. If an object is already moving, a force can change the direction of motion.
Force arrows
You can show the force acting on an object by drawing an arrow. The length of the arrow shows the size of the force. The direction of the arrow shows the direction of the force. The arrow is in contact with the object.
Different types of force
Attracting and repelling
The gravitational force, or force of gravity, is the force that attracts you to the Earth. 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 the object’s weight. This force acts towards the centre of the Earth. It always pulls you ‘down’, wherever you are on the Earth.
A different kind of force is an electrostatic force, which acts between objects that are charged. Rubbing plastic objects can charge them up with static electricity. Once charged they can attract or repel other charged objects. Magnets attract magnetic materials such as iron, steel, or nickel. There is a magnetic force between the magnet and the magnetic material. The objects do not need to be touching to experience the force. It’s the same with gravitational and electrostatic forces.
Forces on moving objects
Friction
Air
If
Upthrust and tension
Measuring forces
You
1 Copy and complete
magnetic force, friction, air resistance, water resistance, thrust, upthrust, and tension are all types of force.
● Forces can change the direction of a moving object, speed it up, or slow it down.
● Forces are measured in newtons using a spring balance.
● Every page starts with the learning objectives for the lesson. The learning objectives match the Cambridge Secondary 1 Science curriculum framework.
● New vocabulary is marked in bold. You can check the meaning of these words in the glossary at the back of the book.
● At the end of each page there are questions to test that you understand what you have learned.
● The key points to remember from the page are also summarised here. These pages cover the Physics topics in the Cambridge Secondary 1 Science curriculum framework. In addition, in every chapter there are also pages that help you think like a scientist, prepare for the next level, and test your knowledge. Find out more on the next page.
A plastic comb attracts pieces of paper with an electrostatic force.
A magnetic force attracts iron filings to a magnet.
Scientific enquiry
These pages help you to practise the skills that you need to be a good scientist. They cover all the scientific enquiry learning objectives from the curriculum framework.
1.5
Questions, evidence, and explanations
Asking questions about the Earth
Have you ever wondered why we don’t fall off the surface of the Earth? Or what pulls us back to Earth when we jump? Investigations often start with questions like this.
Nearly 1000 years ago the Indian mathematician and astronomer Bhaskaracharya asked lots of questions about the Earth and space. He was head of an observatory in Ujjain where he studied the movements of the planets, the Moon, and the Sun. He wondered why the Moon went around the Earth, and why it didn’t get nearer or further away.
Bhaskaracharya noticed that when you drop something, it falls towards the surface of the Earth. He realised that all objects exert a force on other objects –the force that we now call gravity.
He also realised that if the Earth attracted small objects towards it, then it would also attract big objects like the Moon. This is the force, he thought, that keeps the Earth, planets, and Moon in orbit.
Evidence from observation
Bhaskaracharya couldn’t do any experiments to test his idea. He made observations of the world around him, and used the evidence from his observations to come up with an explanation.
Five hundred years later, Sir Isaac Newton thought about the same questions. He knew that a moving object would only change direction if a force acted on it. The Moon had to keep changing direction to stay in orbit around the Earth.
Checkpoint_Physics_ch01.indd 16
You will learn how to:
● consider ideas
● plan investigations and experiments
● record and analyse data
● evaluate evidence to draw scientific conclusions.
7/22/13 8:18 PM
Newton realised that a force must be acting on the Moon to make it do this. Like Bhaskaracharya, he saw that objects fall towards the Earth and wondered if the same force that made them fall kept the Moon from drifting off into space.
Newton published his idea, called the Law of Gravitation, in a book. A few years later, Newton’s law was used to predict the existence of Neptune. In 1846 the planet was discovered as predicted. This was enough evidence for lots of people to believe Newton’s explanation.
Explanations
When there is lots of evidence to support an idea it is usually accepted by other scientists, but it may not be a complete explanation.
Over the last 300 years, lots more evidence about gravity has been collected by scientists. By making predictions and seeing if they match observations, scientists such as Albert Einstein and Edwin Hubble built on Newton’s work. We now know that the force of gravity is much more complicated than both Bhaskaracharya and Newton predicted.
Scientists never stop asking questions and trying to learn more. The more we learn about gravity, the more we can use it. For example, scientists have used the force of gravity to send satellites further out into space than ever before by using the gravitational forces of the Sun and planets to get there.
1 Why could Bhaskaracharya not do any experiments to test his ideas?
2 How did Newton know that there has to be a force acting on the Moon?
3 Give one reason why people might not have believed Bhaskaracharya or Newton when they said that the force of the Sun on the Earth made it move in an orbit around the Sun.
4 Give one reason why people might not have believed Bhaskaracharya or Newton when they said that there is a force on the Moon due to the Earth.
To develop explanations, scientists: ● ask questions ● suggest explanations ● collect and consider evidence.
5 Scientists used Newton’s ideas about gravity to send a spacecraft to the Moon. The Earth pulls on the spacecraft, but the Moon pulls on the spacecraft too! The Moon’s gravity is about one-sixth as strong as the Earth’s gravity. Explain why you need less fuel to get back from the Moon than you need to get there.
You will also learn how scientists throughout history and from around the globe created theories, carried out research, and drew conclusions about the world around them.
Voyager spacecraft has been travelling through the Solar System since 1977, collecting data about the outer planets.
This Indian observatory was built nearly 500 years ago.
Extension
Throughout this book there are lots of opportunities to learn even more about physics beyond the Cambridge Secondary 1 Science curriculum framework. These topics are called Extension because they extend and develop your science skills even further.
You can tell when a topic is extension because it is marked with a dashed line, like the one on the left. Or when the page has a purple background, like below.
Extension topics will not be in your Cambridge Checkpoint test, but they will help you prepare for moving onto the next stage of the curriculum and eventually for Cambridge IGCSE® Physics.
Review
At the end of every chapter and every stage there are review questions.
These questions are written in the style of the Cambridge Checkpoint test. They are there to help you review what you have learned in that chapter or stage.
At the back of this book there are reference pages. These pages will be useful throughout every stage of Cambridge Secondary 1 Science.
Working accurately and safely
Using measuring apparatus accurately
You need to make accurate measurements in science practicals. You will need to choose the correct measuring instrument, and use it properly.
Measuring cylinder
Measuring cylinders measure volumes of liquids or solutions. A measuring cylinder is better for this job than a beaker because it measures smaller differences in volume.
To measure volume:
1. Place the measuring cylinder on a flat surface.
2. Bend down so that your eyes are level with the surface of liquid.
3. Use the scale to read the volume. You need to look at the bottom of the curved surface of the liquid. The curved surface is called the meniscus
Measuring cylinders measure volume in cubic centimetres, cm3, or millilitres, ml. One cm3 is the same as one ml.
Thermometer
The diagram to the left shows an alcohol thermometer. The liquid expands when the bulb is in a hot liquid and moves up the column. The liquid contracts when the bulb is in a cold liquid.
To measure temperature:
1. Look at the scale on the thermometer. Work out the temperature difference represented by each small division.
2. Place the bulb of the thermometer in the liquid.
3. Bend down so that your eyes are level with the liquid in the thermometer.
4. Use the scale to read the temperature.
Most thermometers measure temperature in degrees Celsius, °C.
Balance
A balance is used to measure mass. Sometimes you need to find the mass of something that you can only measure in a container, like liquid in a beaker.
To use a balance to find the mass of liquid in a beaker:
1. Place the empty beaker on the pan. Read its mass.
2. Pour the liquid into the beaker. Read the new mass.
3. Calculate the mass of the liquid like this: (mass of liquid) = (mass of beaker + liquid) – (mass of beaker)
Balances normally measure mass in grams, g, or kilograms, kg.
● how to choose suitable apparatus
● how to work accurately and safely
● how to record, display, and analyse results
● how to use ammeters and voltmeters.
●
●
●
Working safely
Hazard symbols
Hazards are the
Objectives
■ Describe different types of force
■ Understand the effects of forces on moving objects
■ Describe how to measure forces
On a rollercoaster, forces change your speed and direction.
Magnets attract magnetic materials such as iron, steel, or nickel. There is a magnetic force between the magnet and the magnetic material. The objects do not need to be touching to experience the force. It’s the same with gravitational and electrostatic forces. 1.1
Introduction to forces
What is a force?
When a tennis ball hits the ground, a force changes its shape, speed, and direction. A force is a push or pull that can change the shape of an object, or change the way that it moves.
You cannot see forces but you can see what they do. If something starts to move, or speeds up, a force is acting on it. Forces can also slow things down or stop them moving. If an object is already moving, a force can change the direction of motion.
Force arrows
You can show the force acting on an object by drawing an arrow. The length of the arrow shows the size of the force. The direction of the arrow shows the direction of the force. The arrow is in contact with the object.
Different types of force
Attracting and repelling
The gravitational force, or force of gravity, is the force that attracts you to the Earth. 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 the object’s weight. This force acts towards the centre of the Earth. It always pulls you ‘down’, wherever you are on the Earth.
A different kind of force is an electrostatic force, which acts between objects that are charged. Rubbing plastic objects can charge them up with static electricity. Once charged they can attract or repel other charged objects.
A plastic comb attracts pieces of paper with an electrostatic force.
A magnetic force attracts iron filings to a magnet.
Forces on moving objects
Friction is another type of force. When any object slides across a surface, the force of friction tries to stop it moving.
Air resistance is a force that acts on any object moving through the air. Water resistance acts on any object moving through water. Both air resistance and water resistance are types of drag. The moving object collides with the particles in the air or the water, and this slows it down.
In a car or plane, a force called thrust pushes the vehicle forwards.
Upthrust and tension
If an object is floating, the water is pushing it up. This push is a force called upthrust Balloons also experience upthrust because the air below the balloon pushes up.
If you pull something with a rope, there is a force called tension in the rope. There is tension in any rope, wire, cable, or piece of string that has a weight on it.
Measuring forces
You can measure the size of a force. A device for measuring forces is called a forcemeter, such as a spring balance. Forces are measured in units called newtons (N).
1 Copy and complete the table. Tick the correct column for each force: Force Changes the speed of the object Changes its direction gravity acting on a falling apple friction acting on a car going around a corner at a steady speed friction when a car brakes in a straight line
2 Why don’t people on the other side of the Earth fall off?
3 A car is travelling along the road. List three forces acting on it.
4 A magnet can exert a force of attraction or a force of repulsion on another magnet. Which of these forces is a push and which is a pull?
5 a Name a force that is a contact force, which means the object needs to be in contact with something for the force to act.
b Name a force that is a non-contact force.
● Weight, electrostatic force, magnetic force, friction, air resistance, water resistance, thrust, upthrust, and tension are all types of force.
● Forces can change the direction of a moving object, speed it up, or slow it down.
● Forces are measured in newtons using a spring balance.
1.2 Balanced forces
Balanced and unbalanced forces
Objectives
■ Explain the difference between balanced and unbalanced forces
■ Describe the effect of balanced forces
■ Describe the effect of unbalanced forces
The weight of the leaf and the air resistance are balanced. The leaf falls with a steady speed.
You can use forces to explain why an object is moving in the way that it is, or why it is not moving. The force of gravity, or weight, is always acting on the diver. Why is she moving in only one of the pictures?
If the forces on an object are the same size but in opposite directions, then they will cancel out. The forces are balanced. It’s a bit like a tug-of-war when the teams are equal. The object behaves as if there is no force acting on it. When the diver is floating, her weight is balanced by the upthrust of the water. If the forces acting on an object are balanced, then its motion will not change:
● If it is not moving it will stay still.
● If it is moving it will keep moving at a steady speed. If the forces on an object are not equal and opposite, then they are unbalanced. If the forces are unbalanced then the motion of the object will change:
● If it is not moving it will start moving.
● If it is moving it will speed up (accelerate), slow down (decelerate), or change direction.
The forces are unbalanced. The cyclist accelerates. thrust
Lots of people think if something is moving, like a football rolling along the ground, that there is a force acting on it. This is not the case. The football accelerates while it is in contact with the player’s boot. Once it is no longer touching his boot the only forces acting on it come from the things that it is in contact with – the ground and the air. Friction and air resistance will slow it down. If we could remove both of these forces, then the ball would carry on travelling at a steady speed forever. You will learn more about a scientist called Galileo who had this idea on pages 90–91.
Lots of forces
Sometimes more than one pair of forces is acting on an object. Look at the boat. The weight and the upthrust are balanced. The boat does not move up or down.
The thrust of the engine is balanced by the air and water resistance. The boat moves forwards with a steady speed. You know that if an object is stationary or moving at a steady speed, then the forces on it are balanced. If it is speeding up or slowing down, then the forces acting on it will be unbalanced.
Resultant forces
If you know the size of each force acting on an object, you can work out the resultant force. If the arrows are in the same direction you add the forces. If they are in opposite directions you take away one force from the other.
● If the resultant force is zero the forces are balanced
● If the resultant force is not zero the forces are unbalanced
1 A cricketer drops a ball and it speeds up. Are the forces on it balanced or unbalanced?
2 A boy pushes a toy car along the floor at a steady speed. Are the forces on it balanced or unbalanced?
3 Arzu is throwing a ball up in the air and catching it. Copy and complete the sentences using the words ‘balanced’ or ‘unbalanced’.
a When Arzu is holding the ball and it is not moving, the forces on the ball are … .
b When the ball is moving upwards and slowing down, the forces on it are … .
c When the ball is moving downwards and speeding up, the forces on it are … .
4 Extension: Vil says that if a 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?
● Forces are balanced if they cancel each other out. They are unbalanced if they do not cancel out.
● An object stays still or moves at a steady speed if the forces on it are balanced.
● Unbalanced forces change the speed or direction of a moving object.
The resultant force is 4 N to the right.
The resultant force is 8 N to the left.
The resultant force is zero.
Objectives
■ Describe the effect of friction on moving objects
■ Understand how to reduce friction
■ Describe how friction can be useful
Friction
What is friction?
Friction is a force that slows down moving objects.
When a child goes down a slide, there is a force of friction between them and the slide. This is because all surfaces, even surfaces that feel very smooth, are uneven. A metal slide looks smooth, but under the microscope you can see how uneven it is.
You need to push things to get them to move. It is the uneven surfaces that produce the force of friction that you have to overcome.
Even smooth surfaces like shiny metal are rough if you look at them under a microscope.
Cyclists put oil on the axles of their bicycle to reduce friction. This is called lubrication. A layer of oil between two surfaces makes it easier for the surfaces to slide over each other. It reduces the force of friction. 1.3
Reducing friction
People often try and reduce friction, for example skiers wax the bottoms of their skis to reduce friction and make them go faster.
There are ball-bearings inside the wheels of a skateboard. They roll over each other as the wheel turns, and reduce friction. This means that the surfaces are not worn away as quickly.
Friction can be useful
Friction is not always a bad thing. When you walk friction is useful. Friction acts between your feet and the ground, making it possible for you to move. Vehicles also need the force of friction between the tyres and the road to make them move. In icy conditions friction is reduced and the wheels skid because there is not enough friction for the tyres to grip the road. When there is not much friction, you realise how important friction is!
Bicycles and cars also need friction to stop. The force of friction acts between their brakes and the wheels. When surfaces rub against each other like this they get warm, just like when you rub your hands together. The temperature of brake pads can get very high if the car is moving fast and it brakes hard. Sometimes they get so hot that they glow. The friction between brake pads and the wheels wears them away and they have to be replaced.
1 What causes the force of friction between two surfaces that are sliding over each other?
2 Explain how using oil for lubrication reduces friction.
3 a There is a thin layer of water between the blade and the ice. Suggest how this affects friction and helps the skater.
b Suggest why ice skates have a jagged edge at the front.
4 Tyres have treads to remove water from between the tyre and the road. Why is it important to remove the water?
● The force of friction slows things down.
● You can reduce friction by lubrication or by using ball-bearings.
● Friction is useful for grip, to start moving, or for braking.
1.4 Gravity
The force of gravity
Objectives
■ Explain the link between gravity, mass, and weight
■ Describe how your weight can be different on different planets
There is a force between all objects called the force of gravity. This force is pulling you down towards the centre of the Earth. But forces come in pairs – you are also pulling the Earth with a force of gravity. The size of the force of gravity between two objects depends on:
● the mass of the two objects
● how far apart they are.
Weight and mass
The force of the Earth’s gravity on an object is called its weight. When you use scales or a forcemeter you are measuring the force of gravity.
There is a force of gravity between you and the Earth
Weight is a force so it is measured in newtons (N). Mass is related to the amount of matter in an object and it is measured in kilograms (kg). Another way of thinking about mass is to do with motion. Something with a small mass will accelerate faster when you apply a force to it than something with a big mass.
It’s easy to confuse weight and mass. Your weight is a force and should be measured in newtons, but often scales measure your ‘weight’ in kilograms! Scientifically this is wrong.
Your weight can change
The force of gravity is different on every planet. The more massive the planet, the larger the force of gravity. If you could visit different planets your weight would change, but your mass would stay the same.
On the Moon there is no atmosphere (air) and astronauts have to wear spacesuits to be able to breathe. Some people think this means there is no gravity there. This is not true. There is a force of gravity on the Moon, but it is about one-sixth as strong as on Earth.
Gravitational field strength
There is a gravitational field around all objects. A gravitational field is a region where a mass experiences a force.
On Earth the gravitational field strength is 10 newtons per kilogram (N/kg). To work out the weight of an object use this equation:
Weight = mass × gravitational field strength (N) (kg) (N/kg)
When people say that gravity is different on different planets, they mean that the gravitational field strength is different.
In physics the idea of a ‘field’ is very important. Fields are linked to forces. There are other types of field, such as magnetic fields (see page 138).
1 Explain the difference between weight and mass.
2 A baby has a mass of 4 kg. What is its weight on Earth?
3 Is the mass of the astronaut on Mars bigger, smaller, or the same compared with on Earth? Explain your answer.
4 A student says that objects get pulled down because the Earth is like a big magnet.
a How is the gravitational force like a magnetic force?
b How is the gravitational force not like a magnetic force?
5 Extension: the gravitational field strength on Mars is 4 N/kg. Calculate the weight of the astronaut on Mars.
● The force of gravity on an object on Earth is called its weight. It depends on the mass.
● Weight is a force, measured in newtons.
● Mass is the amount of matter, measured in kilograms.
● The force of gravity is different on different planets or on the Moon.
The weight of the astronaut changes, but his mass stays the same.
1.5
Objective
■ Discuss the importance of questions, evidence, and explanations
Questions, evidence, and explanations
Asking questions about the Earth
Have you ever wondered why we don’t fall off the surface of the Earth? Or what pulls us back to Earth when we jump? Investigations often start with questions like this.
Nearly 1000 years ago the Indian mathematician and astronomer Bhaskaracharya asked lots of questions about the Earth and space. He was head of an observatory in Ujjain where he studied the movements of the planets, the Moon, and the Sun. He wondered why the Moon went around the Earth, and why it didn’t get nearer or further away.
Bhaskaracharya noticed that when you drop something, it falls towards the surface of the Earth. He realised that all objects exert a force on other objects –the force that we now call gravity.
He also realised that if the Earth attracted small objects towards it, then it would also attract big objects like the Moon. This is the force, he thought, that keeps the Earth, planets, and Moon in orbit.
Evidence from observation
Bhaskaracharya couldn’t do any experiments to test his idea. He made observations of the world around him, and used the evidence from his observations to come up with an explanation.
Five hundred years later, Sir Isaac Newton thought about the same questions. He knew that a moving object would only change direction if a force acted on it. The Moon had to keep changing direction to stay in orbit around the Earth.
This Indian observatory was built nearly 500 years ago.
Newton realised that a force must be acting on the Moon to make it do this. Like Bhaskaracharya, he saw that objects fall towards the Earth and wondered if the same force that made them fall kept the Moon from drifting off into space.
Newton published his idea, called the Law of Gravitation, in a book. A few years later, Newton’s law was used to predict the existence of Neptune. In 1846 the planet was discovered as predicted. This was enough evidence for lots of people to believe Newton’s explanation.
Explanations
When there is lots of evidence to support an idea it is usually accepted by other scientists, but it may not be a complete explanation.
Over the last 300 years, lots more evidence about gravity has been collected by scientists. By making predictions and seeing if they match observations, scientists such as Albert Einstein and Edwin Hubble built on Newton’s work. We now know that the force of gravity is much more complicated than both Bhaskaracharya and Newton predicted.
Scientists never stop asking questions and trying to learn more. The more we learn about gravity, the more we can use it. For example, scientists have used the force of gravity to send satellites further out into space than ever before by using the gravitational forces of the Sun and planets to get there.
1 Why could Bhaskaracharya not do any experiments to test his ideas?
2 How did Newton know that there has to be a force acting on the Moon?
3 Give one reason why people might not have believed Bhaskaracharya or Newton when they said that the force of the Sun on the Earth made it move in an orbit around the Sun.
4 Give one reason why people might not have believed Bhaskaracharya or Newton when they said that there is a force on the Moon due to the Earth.
5 Scientists used Newton’s ideas about gravity to send a spacecraft to the Moon. The Earth pulls on the spacecraft, but the Moon pulls on the spacecraft too! The Moon’s gravity is about one-sixth as strong as the Earth’s gravity. Explain why you need less fuel to get back from the Moon than you need to get there.
To develop explanations, scientists:
● ask questions
● suggest explanations
● collect and consider evidence.
The Voyager spacecraft has been travelling through the Solar System since 1977, collecting data about the outer planets.
In 1969 the first person landed on the Moon and was able to see this view of Earth, which had never been seen before.
Objectives
■ Explain what affects air resistance
■ Describe what is meant by terminal velocity
■ Explain how and why the speed of a skydiver changes
Air resistance
What is air resistance?
The air can produce a force that changes your face!
When an object moves through air, there is a force on it called air resistance. Most of the time you don’t really notice it. If the air is moving very fast, or you are moving very fast through the air, then you notice the effect.
An object moving through the air collides with the particles in the air and is effectively pushing the air out of the way. These collisions with air particles provide the resistance.
Air resistance is also affected by the speed of the object moving through the air. Objects moving with a higher speed will push more air out of the way and experience more air resistance.
Reducing air resistance
Air resistance can be a problem. It is a form of friction that slows things down. Air resistance is less if the area in contact with the air is reduced. Streamlining reduces air resistance by 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. 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.
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 parachutist jumps out of a plane she accelerates because of the force of gravity acting on her. As she speeds up the air resistance increases until it balances her weight. She then falls with a steady speed called the terminal velocity.
Smoke shows the path of air in a wind tunnel.
Rocket cars use parachutes for braking.
When she opens her parachute the air resistance suddenly increases. This slows her down. As she slows down the air resistance decreases until she reaches a new, slower, terminal velocity and can land safely.
Jumps Lands
This graph shows how the speed of a parachutist changes as she falls.
Do heavier things fall faster?
When we see things fall on Earth, like a feather or a hammer, the heavier object (the hammer) falls faster. This is because of air resistance. Air resistance affects the motion of the feather much more than the motion of the hammer. If you remove the air, or do the experiment somewhere where there is no air like the Moon, the hammer and feather hit the ground at the same time. If there is no air resistance all objects fall at the same rate.
1 Car manufacturers put cars in wind tunnels to help them to design streamlined cars.
a Explain what is meant by ‘streamlined’.
b Which would experience more air resistance – a streamlined car travelling slowly or a lorry travelling fast? Explain your answer.
2 Copy and complete these sentences about the forces on a skydiver jumping out of a plane using the words ‘balanced’ or ‘unbalanced’. When the skydiver jumps out of the plane the forces are . When she reaches a steady speed of about 50 m/s the forces are . The parachute then opens and this makes the forces . She then reaches a lower terminal velocity of about 5 m/s and the forces are . Finally the skydiver lands and stands on the ground. The forces are
3 Explain why a tennis ball and a cricket ball dropped together will hit the ground at about the same time, even though the cricket ball is heavier.
● Air resistance depends on the speed of the object and its area in contact with the air.
● The shape of streamlined objects reduces air or water resistance.
● Objects fall at terminal velocity when their weight is balanced by air resistance.
● A parachute reduces the terminal velocity so a skydiver can land safely.
1.7 Planning investigations
Asking questions
Objective
■ Understand how to plan an investigation to test an idea in science
How does the mass affect the time to
does the
Kasini was watching a film about dolphins. The dolphins have to swim fast to catch fish. She wondered what affects how fast things can move through water.
Ideas to test
Kasini decided to make different objects out of clay and drop them into a cylinder of water. She could time how long they took to hit the bottom. These are some of the ideas that she thought about testing. Investigations are ways of obtaining evidence. 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 the object affects how long it takes to fall through water.
In an investigation something that can be changed is called a variable. This is the list of possible variables that Kasini thought of:
● the shape of the clay
● the mass of clay
● 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 clay. It is very important to change only one variable at a time. She decided that she would use the same mass of clay and the same volume of water at the same temperature each time.
Making a prediction
Kasini had learned that engineers design cars and aeroplanes to be as streamlined as possible to reduce drag. She knew that air resistance is a form of drag, and so is water resistance. She used this information to make a prediction.
I predict that the cone shape will reach the bottom in the shortest time. This is because there will be less water resistance because a cone is a streamlined shape.
How
shape affect the time to hit the bottom?
hit the bottom?
Making a plan and choosing equipment
This is Kasini’s 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, cube, sphere, cylinder, cuboid.
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 measuring cylinder
• modelling clay
• a stopwatch
• a balance
• a measuring jug.
I will write my results in a table.
Making improvements
Kasini completed her investigation and wrote down her results. She discussed her investigation with Nadia. Nadia asked if there were any problems with the investigation. Kasini said that it was difficult to see exactly when to start and stop the stopwatch because all the shapes moved quickly through the water.
1 Copy and complete this table to explain why Kasini needed each piece of equipment.
Equipment
Why Kasini needed it a large measuring cylinder modelling clay a stopwatch a balance a measuring jug
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 could the plan be improved to give better results?
4 What has Kasini missed out of her results table?
5 Nadia says that two variables that Kasini should have in her list are the type of stop clock and the type of clay. Do you agree? Explain your answer.
● An idea can be tested by carrying out an investigation.
● Variables are things that you can control, change, or observe.
● You can use scientific knowledge to make predictions.
Objectives
■ Describe what happens when you stretch a spring
■ Explain what is meant by tension
■ Explain the elastic limit
■ Explain why things float or sink
Tension and
upthrust
Under tension
Ropes for rock climbing are designed to stretch a certain amount, just in case the climber falls. If the rope did not stretch then he could be injured. Ropes, cords, and springs all stretch when you pull them.
There is force in the rope that balances the climber’s weight. The force in a rope that is being stretched is called tension. As you pull on the rope the particles inside are moving apart. The tension force that you feel is the force of attraction pulling the particles back again.
Elastic bands and bungees
A bungee cord is designed to stretch a very long way. It is made of lots of elastic cords all bound together. If something is elastic, it will go back to its original length when you remove the force. The amount that it stretches is called the extension
If something is not elastic, and does not go back to its original length after stretching, we say it is plastic
Stretching a spring
Springs have lots of uses. They are used in spring balances, trampolines, and car suspension systems. It is important to use the right spring for the job, so you need to find out how much a spring will stretch when you apply a force to it.
A bigger force will produce a bigger extension, and if you double the force you double the extension. The extension is proportional to the force. This is why we use a spring balance to measure forces. The spring is elastic.
If you keep loading more and more on, eventually the spring will not return to its original length when you remove the force. It has reached its limit, called the elastic limit. The spring cannot spring back and it is permanently extended
Elastic materials will break if the force applied to them is too big.
Trampolines need springs that do not stretch very much.
Why do things float?
It seems amazing that something as large and heavy as an oil tanker can float.
When the oil tanker is in the water there is a force pushing up on it called upthrust. The upthrust balances the weight so the oil tanker floats. The water particles collide with the bottom of the boat and push it up. You can feel the force of upthrust if you try to push a balloon into a bowl of water.
If you weigh an object underwater it appears to weigh less because of the upthrust. It is much easier to pull yourself out of a swimming pool than it is to pull yourself up when you are not in water.
In this diagram, the upthrust is 4 N. The water level has risen because the weight has displaced some water. The weight of the displaced water is 4 N. This is Archimedes’ principle. The apparent weight of the object is now 6 N.
Floating in air
A balloon filled with helium will rise. There is an upwards force on the balloon because there are more collisions between the air and the balloon at the bottom of the balloon than above it. This is because there are more air molecules lower down due to gravity. This upwards force produces an upthrust on the balloon. Its motion is changing so there must be a resultant force on it. When the upthrust is bigger than the weight of the balloon and the air resistance, the balloon will accelerate. When the forces on it are balanced it will move with a steady speed.
1 A spring is 3 cm long. A student hangs a weight of 2 N on it. It is now 4.5 cm long.
a What is the extension?
b What would be the extension with a weight of 4 N?
c What would be the length of the spring with a weight of 6 N?
2 A floating boat weighs 20 000 N. What is the size of the upthrust?
3 Sketch the diagram of the 10 N weight underwater. Draw and label the forces on it.
4 Explain what is meant by the elastic limit of a spring.
5 When people do a bungee jump they are asked how much they weigh. Why?
● Springs, ropes, and elastic materials have a tension in them when stretched.
● The extension is proportional to the tension, up to the elastic limit.
● Springs in tension pull back.
● Objects float when the upthrust equals the weight.
1.9 Presenting results – tables and graphs
Objectives
■ Describe how to present results in tables
■ Describe how to draw line graphs
■ Explain what is meant by continuous variables
The elastic limit Suma is on the trampoline. The label says that the weight limit for the trampoline is 1200 N. She works out that anyone with a mass of 120 kg or less can use it. Someone with a mass bigger than 120 kg could be injured on the trampoline. This is because the springs could be permanently extended or even break.
She thinks that for safety, 1200 N must actually be much, much smaller than the elastic limit.
Recording the results
How can I find out the elastic limit of a spring?
Suma collects a spring, some weights, and a metre ruler. She draws a table ready for her results.
Weight (N) Extension (cm)
Suma measures the length of the spring with no weights on it. This is the original length. There is zero extension, so she records that in her table. She adds one 1 N weight to the spring and measures the new length. To find the extension she subtracts the original length from the new length. She writes the result in her table.
