1

PHYSICS

Astronomy

The Solar System and beyond We live on a planet called the Earth. The Earth gets heat and light from the Sun. The Earth spins on its axis once every 24 hours. The side of the Earth facing the Sun has daylight, and it is night on the side facing away from the Sun.

The Earth orbits around the Sun. It takes one year to go around once. A year is actually 365.25 days long, so every four years we have a leap year, when an extra day is added. The Moon is a satellite of the Earth. It orbits the Earth once every 28 days. This is called a lunar month. We can see the Moon because it reflects light from the Sun. The Moon seems to change shape during the month. The different shapes are called phases of the Moon. The phases happen because we cannot always see all of the part that is lit by the Sun. Sometimes the Moon blocks the light from the Sun. When this happens we get a solar eclipse. If the Moon goes into the shadow of the Earth we get a lunar eclipse.

2

The Earth’s axis is tilted. When the northern hemisphere is tilted towards the Sun it is summer in the UK. Days are longer than nights, and the Sun is higher in the sky. The Sun’s rays are more concentrated, so it feels hotter.

There are nine planets orbiting the Sun, and lots of asteroids. Most of the planets have moons orbiting around them. The Sun, the planets and their moons, and the asteroids make up the Solar System. The nine planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. You can remember the order of the planets using this sentence ‘My Very Easy Method Just Sums Up Nine Planets’.

Planets do not make their own light. We can sometimes see the planets because they reflect light from the Sun. The Sun is a star. It is a ball of gas that gives out large amounts of heat and light energy. The Sun is like the stars you can see in the sky at night. The stars do not look very bright because they are a lot further away than the Sun. People often group stars into patterns called constellations. The Sun is one of millions of stars in our galaxy, which is called the Milky Way. There are millions of galaxies in the Universe. The stars are a very long way from Earth. Scientists measure distances to the stars using light years. A light year is the distance that light can travel in one year.

3

1. What is the name of the closest star to us? The sun

2. What is the name of an object that orbits a star? A planet

3. What is the name of a rock that orbits a planet? A Moon

4. What is the name of a man made object that orbits a planet? Satellite

5. These objects have several functions, name four of them.

Spy satellites to photograph the enemy,

Communications satellites for phone and TV,

Weather satellites to see the weather patterns,

Telescopes to look into space

6. How long does it take the Earth to go round the sun? 1 year 7. How long does it take for the Earth to rotate once 1 day 8. How long does it take for the moon to go round the Earth? 28 days 9. Name the planets in order.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto

10. Which ones are solids? First 4 and the last 11. Which ones have moons? Earth to Pluto 12. What happens to the temperature of the planets as they get further

away from the Sun? It gets colder

13. Which planet does not fit into this pattern? Venus Why is it different? It has an atmosphere which keeps it hotter than Mercury

14. During the Summer in England:- a) What season is it in Australia? Winter b) Do we have longer nights or days? Longer days c) Does the Sun appear higher or lower in the sky? Higher d) Which country would be in perpetual day? Scandinavia e) Does the sun shine directly or at an angle? It is more direct

15. Will a solar eclipse happen in the day or at night?

Day, the moon comes between the earth and the sun

4

16. Will a lunar eclipse happen in the day or at night? Night the earth comes between the sum and the moon

17. Draw a diagram of the Sun Earth and Moon in the position for a Solar eclipse.

Sun Sun Moon Earth Eclipse 18. Put the following in order with the smallest first.

Milky Way Galaxy, Earth, Universe, Sun, Moon, Solar system, Satellites, Andromeda group.

Satellites, Moon, Earth, Sun, Solar system, Milky way galaxy, Andromeda group, Universe

19. What holds the planets in orbit around the Sun? Gravity 20. Is this force greater or smaller between the Earth and the Moon?

Greater on Earth than on the Moon

21. Why do we see the Sun? It generate light 22. Why do we see the Moon? It reflects light off the sun Magnets

1. A magnet has an invisible force field around it. Draw a diagram.

2. Two North seeking ends will do what? Repel 3. A north-seeking end will do what to a South-seeking end? Attract

5

4. A compass has a magnet as a pointer. Draw a compass near your magnet and show where the needle is pointing.

5. How could you show that your magnet was stronger than mine?

By seeing how many nails it could pick up.

6. How can electricity be made to create a magnetic field? If the electricity flows round the nail in insulated wire, the nail becomes a magnet

7. How could you show that a mystery object was a magnet? Float it and see if it points North

8. Name all the magnetic elements you know. Iron, Cobalt & Nickle Pressure We can work out the pressure on something by using this formula:

pressure = force ÷ area

Pressure can be measured in: • newtons per square metre (N/m2)

• newtons per square centimetre (N/cm2)

• pascals (Pa).

1 Pa = 1 N/m2

9. How do you work out Pressure? What is the unit? P =F ÷ A in N/cm2

10. From the triangle, how could you work out area? A = F ÷ P

What is the unit of measurement for area? Cm2 11. From the triangle, how could you work out Force?

F = P x A What is the unit? N

12. How do you find your force pushing down on the ground if all you have is a set of bathroom scales measures in kg? Measure in Kg and convert to Newtons by multiplying by 10 (1Kg = 10N)

13. Give an example of the pressure being made large by reducing the area. Making a sharp point

6

14. Give an example of the pressure being made small by enlarging the area. Caterpillar tracks

Sound 1. Sound travels at approx 300m/sec. through air. How long before you hear the echo of a

starting pistol coming off a building 1200m away?

S=D ÷ T

so time =D ÷ S

1200 ÷ 300 = 4sec each way so with the echo 8 sec

2. How does sound travel through the air? The air is moved in waves 3. How does sound travel through a solid object?

The particles vibrate each other

4. Does it travel faster or slower through a metal like steel? Faster 5. What happens to sound in a vacuum?

No molecules present so sound cannot travel through.

6. The pitch of a note is sometimes called its what? Frequency

7. The pitch is higher if the waves are closer together or further apart?

Closer together

8. Pitch is measures in what unit? Hz 9. Name an animal which hears at a higher pitch than you.

Dog, bat, whale

10. The volume is sometimes known as its what? Amplitude 11. The higher the wave the louder or quieter the sound? Louder 12. Give an example which proves that sound travels slower than light.

You see lightening then hear the thunder

7

13. How can you make a guitar make a higher note? (2 ways) Shorten or tighten the string or make the string thinner

14. Give an example of how sound can be used to measure distance. Radar, ultra sound

Electrical circuits Electricity is a flow of electrons. Electricity can flow through conductors but not through insulators. Metals are good conductors of electricity. Circuits A complete circuit is needed for electricity to flow. The current is the amount of electricity flowing in the circuit. The units for current are amps (A). Current is measured using an ammeter. The resistance of a circuit is a way of saying how easy or difficult it is for electricity to flow. • high resistance = hard for electricity to flow = small current

• low resistance = easy for electricity to flow = large current

Circuits can be series or parallel circuits.

Series circuit Parallel circuit

• If one bulb breaks, all the others go off.

• The current is the same everywhere.

• If you put more bulbs in they will be dimmer, because it is harder for the electricity to get through. The resistance of the circuit is higher.

• The voltage from the cell or power pack is divided between the components.

• If one bulb breaks, the bulbs in the other branches stay on.

• The current splits up when it comes to a branch. The current in all the branches adds up to the current in the main part of a circuit.

• If you add more bulbs they stay bright. It is easier for the current to flow with more branches, because there are more ways for the electrons to go.

• The voltage is the same across all the branches of the circuit.

8

Electricity and heat • When electricity flows through a wire, the wire can get hot.

• Hot wires are used in electric fires, irons and cookers.

• A fuse is a thin piece of wire that melts if too much electricity flows through it. It is used for safety.

Voltage A circuit must have a cell or power supply to provide a voltage. The voltage pushes the electrons around the circuit and gives them energy. The voltage of a cell can be measured using a voltmeter. The units for voltage are volts (V). The voltage across a component is a way of measuring how much energy the component is transferring. The voltage across all the components in a series circuit adds up to the voltage across the cell.

A wire with electricity flowing through it has a magnetic field around it. An electromagnet is a coil of wire with an electric current flowing through it. You can make an electromagnet stronger by: • increasing the number of coils of wire

• increasing the size of the current (by increasing the voltage)

• using an iron core.

Electromagnets can be used for lifting things. They are also used in electric bells, relays and in video and music recording. Electromagnets are used to make bells work.

9

A reed switch has two thin pieces of iron inside it. If a magnet is held near the switch, the pieces of iron are magnetised and touch each other. A reed switch can also be switched on using an electromagnet. Any switch that is worked by electricity is called a relay.

1. What types of substances allow an electric current to flow through them, conductors or insulators? Conductors

2. Give examples of materials that conduct electricity. Any metal or carbon

3. What pushes the electricity round a circuit? A cell produces a voltage which pushes the electrons round

4. What is the unit used to measure the push? Voltage 5. What instrument measures this? Voltmeter Using voltmeters When you are building a circuit with voltmeters, it is easier if you ignore the voltmeters to start with. Build this first: Then add the voltmeters:

6. Draw a circuit diagram of a circuit where this is being measured and show the result. 1.5v 7. What unit is used to measure the flow or current of electricity in a circuit? Amps 8. What instrument measures current? Ammeter

10

9. Draw a circuit measuring the current and show the result. 0.2a 10. In which two ways can bulbs be connected in a circuit?

Series and parallel

11. Which method makes the bulbs brighter? Parallel 12. Which circuit takes the higher current? Parallel 13. Draw two circuits to show these two types of circuit and mark on ammeter readings.

0.4a

0.2a 0.2a 0.2a 0.1a 0.2a 14. If you increase the number of bulbs or resistance, what happens to

the current? It gets less

15. If you make the wire thicker, what happens to the current? It increases (more room to flow)

16. If you add more cells, what happens to the current? It doubles 17. If you increase the length of a wire, what happens to the current.

It gets less

18. Draw circuit diagrams for the following and say what they do.

11

Component Symbol What does it do?

cell

Cell – pushes the electricity round a circuit

battery of cells

bulb

Lamp- Light up when a current passes through

switch

Closes and completes the circuit

resistor

Reduces the amount of current in a circuit

Light Emitting Diode

LED

LED Only allows the current to flow one way (with the arrow) and lights up using very little current.

Coil

Forms an electromagnet when current

passes through.

Reed Switch

A switch that is closed by a magnet

Buzzer

Buzzer – it buzzes

variable resistor

Variable resistor:- As the knob it turned the resistance gets higher (less current) or lower (more current). Eg a dimmer switch or a volume control

motor

It turns a motor

ammeter

Measures Current

fuse

Fuse – Melts if too much current passes through and breaks the circuit

12

Light Dependent Resistor

LDR

LDR In the light it has a low resistance and allows a larger current to flow through. (the bulb would be brighter) In the dark it has a high resistance allowing only a small current through.

Push Switch

Closes the circuit when pushed and held.

SPDT Switch

2 way switch – Can switch the current from one circuit to another

Reed Relay

19. Draw a circuit where two switches are connected so as to create an AND gate. 20. What might this be used for?

Safety switch needing both to be switched to make it work.

21. Draw a circuit where two switches are connected so as to create an OR gate.

22. What might this be used for?

Either switch or both will switch it on ie a door bell worked from two places.

13

23. Explain with the use of a diagram how an electromagnet works. Circuit A Coil switch Circuit B 24. Explain with the use of a diagram how an electromagnetic relay works.

The LDR in circuit A allows enough current through to magnetise the coil. This magnet closes the switch which turns on circuit B

Generating electricity Fossil fuels are transported to power stations where they are burnt to release heat energy. This heats water, turning it to steam. The steam drives turbines, which turn generators. The electricity generated flows along cables into the National Grid.

Measuring Volumes

If a solid has a regular shape, you can measure it with a ruler. You can then work out its volume using this equation: Volume = length x width x height

If the solid does not have a regular shape, working out its volume is a bit more difficult. If the solid is small, you can measure its volume using a measuring cylinder. Put some water in the measuring cylinder and write down the volume. Then carefully drop the solid into the water, and write down the new volume. The difference between the two volumes is the volume of your solid.

Volume of object = volume with object – volume without object

MM

m

14

If the solid is too big to fit into a measuring cylinder, you need to use a displacement can. The displacement can is full of water.

The volume of water that the stone has pushed out of the can is the same as the volume of the stone.

If your solid object floats, you will have to push it down until it is just under the water.

15. How do you measure the volume of a regular shaped object?

L H W

LxBxH What is the unit? Cm3

16. How do you measure the area of a regular shaped object? LxW

What is the unit? Cm2

15

17. How do you measure the volume of an irregular shaped object?

Put it into a eureka beaker and measure the water that flows out in a measuring cylinder What is the unit? Cm3

18. How do you measure the volume of a liquid?

Pour it into a measuring cylinder What is the unit? ml or l (Litres and millilitres)

19. How do you measure the internal volume of a bottle? Fill it with water and pour into a measuring cylinder

What is the unit? Cm3 or ml Particle theory 1. Draw the particles in a solid, a liquid and a gas.

solid liquid gas 2. How may a solid be changed into a liquid? Heat 3. How may a liquid be turned into a gas? Heat or decompress 4. Can you compress a gas into a liquid? Yes eg liquid oxygen 5. Can you compress a liquid into a solid?

No the particles are too close together

6. At what temperature does water condense? 1000C 7. At what temperature does ice melt? 00C 8. As water turns into steam, what happens to its volume?

It gets much bigger

9. Which is heavier a tonne of lead or a tonne of feathers? Both the same, a tonne

10. Which takes up the most space?

16

A tonne of feathers would take up a huge space

11. Lead is D_______ than feathers. Denser 12. Draw the D triangle.

M

D V Remember the formula for calculating density is:

Density = mass

volume = mass ÷ volume

Material Mass (g) Volume (cm3) Density

Copper 1800 200

Wood 900 1500

Lead 567 50

Iron 790 100

Water 1000 1000

Polythene 690 750

Glass 197 80

Plasticine 26 20

Bone china 42 15

Brick 3200 2000

13. How do you work out density? M ÷ V

What is the unit? g / cm3

14. How do you find the mass? Weigh it 15. How do you find the volume? Measure it LxBxH

17

16. Which will be denser if any, a small block of Aluminium or a large block of Aluminium? Aluminium has the same density 2.8g/cm2 no matter what its size

17. Water is denser than oil, so what would happen if you mixed water and oil? Oil will float on water

18. Water’s density is 1g/cm3. You just float on water, what is your density therefore? Just less than 1g/cm

19. A block of wood has a mass of 250g, its volume is 500 cm3, what is its density. Don’t forget the unit.

D = M ÷ V 250 ÷ 500 = 0.5 g/cm3

Light

1. Name the three primary light sources. Sun, Reactions, Electrical 2. What does opaque mean? Light cannot pass through 3. What does translucent mean, give an example?

Light passes through but not clearly

4. What does transparent mean, give an example? Light passes through clearly

5. What does luminous mean? Give an example from astronomy. Produces light eg a star 6. How can you show that light travels in straight lines?

Beams in a dusty room.

7. Draw a plane mirror and show how a ray of light may be reflected off it. (don’t forget the arrows)

8. Draw in the Normal. 9. Mark the angle of incidence and the angle of reflection. 10. What rule does reflection of light obey?

The light comes out at the same angle that it went in. I=r

18

11. Where might you find a concave mirror and what would it do? Face mirror, it makes the image larger

12. Where might you find a convex mirror and what would it do? Security mirror or car mirror, you see more but the image is smaller

13. Explain how we see things with a diagram. (NB you do not need to know how the eye works)

14. Why does an object look red? All the colours of the spectrum are absorbed but red which is reflected into your eye.

15. Why does an object look white? All the colours are reflected and none are absorbed. All colours together make white.

16. Why does an object look black? All the colours are absorbed and none are reflected so you see nothing ie black.

17. Why wear black clothes in the winter and white ones in the summer? White reflects all the sun so you stay cool. Black absorbs all the sun so you get hot

18. When light passes into a glass block it bends towards the normal. As it leaves it bends away from the normal. Draw a diagram to show this.

19

19. What is this bending of light called? Refraction 20. Draw a single red light ray entering and leaving a prism. 21. Draw a beam of white light entering a prism and splitting as it leaves. Add the colours.

Conduction, Convection & Radiation The kinetic theory or particle model of matter helps to explain how some forms of heat energy travel. The theory suggests that everything is made of moving or vibrating particles. When these particles are heated they move faster.

20

When the particles vibrate faster the material expands. When the air particles in the balloon are heated, they move apart and the air expands and becomes less dense. This causes the hot air to rise, and the balloon rises too.

Thermal energy can travel in three different ways. Conduction takes place in solids and can also happen in liquids (although not very well).

The particles in a solid are held together tightly.

When they gain energy they vibrate faster and the vibrations are passed on. Particles are not as close in a liquid, so conduction is not very good.

Metals are the best conductors. Most other solids are poor conductors.

Something which does not conduct heat very well is an insulator.

Liquids, gases, and solids which contain a lot of trapped air are insulators. Convection takes place in liquids and gases.

When the air near the fire is heated, the particles spread further apart and the air becomes less dense and rises.

As it rises it meets cooler air and passes the energy on.

Having passed on the energy, it cools and becomes denser.

The denser air sinks, setting up a cycle or convection current.

21

Heat can be transferred through empty space by infrared radiation. Radiation does not require the movement of particles. Any hot or warm object gives off or emits radiation. When something takes in heat energy from radiation, it is said to absorb it.

Infrared radiation travels as waves. It can be reflected and it can also be focused. Changes of state Substances can change state when they are heated or cooled.

The melting point and the freezing point of a substance are the same temperature.

The temperature of a substance does not change while it is melting, even if it is still being heated.

Energy

Nothing would happen without energy. Energy is needed to: • keep our bodies working

• make machines work

• heat homes, schools and offices.

Energies in action • heat energy

• light energy

• sound energy

• electrical energy

• kinetic (movement) energy.

22

Stored energy Some energy has to be stored so that it is ready for use when we need it. • Chemical energy is stored in food, fuels and cells.

• Gravitational potential energy is stored in high up things.

• Strain energy is stored in stretched or squashed things.

• Nuclear energy is stored inside atoms.

How is energy measured? Energy is measured in joules (J) or kilojoules (kJ). A kilojoule is 1000 joules. Energy changes Energy needs to be changed to be useful.

in the cells in the wires in the bulb

An energy flow diagram.

This electrical energy is transferred to other components in the circuit, which convert it to other forms of energy. For instance, a light bulb transfers electrical energy to heat and light energy.Many energy changes take place in everyday life. Often wasted energy is produced in the forms of heat or sound.

A car engine produces kinetic energy, which is useful. It also produces heat and sound

Energy cannot be made or destroyed, but can only be changed from one form to another. This is the law of conservation of energy. Energy from the Sun Most of the energy resources we use originally came from the Sun. Only geothermal energy, nuclear power and tidal power do not depend on energy from the Sun.

23

We need fuels to provide energy in our homes, factories and for transport. A fuel is something which can release heat energy. Fossil fuels Fossil fuels: • are made from plants and animals which were trapped in mud and rocks millions of

years ago

• include coal, oil and natural gas

• are non-renewable (they take millions of years to form, and so our supplies will run out)

• produce gases which cause pollution when they are burnt

• are relatively cheap to obtain

• contain chemical energy which changes to heat energy when they are burnt

• originally got their energy from the Sun. The plants that became coal got their energy from the Sun, and the animals that became oil got their energy from plants which got their energy from the Sun.

Electricity is not a fuel. It has to be generated using other energy resources. Renewable energy resources Renewable energy resources: • include solar, wind, tidal, wave, biomass, geothermal and hydroelectricity

• do not produce harmful gases

• can be expensive

• will not run out.

24

1. Name three primary sources of energy. Sun, chemical reactions, nuclear

2. Name 6 types of energy. Kinetic, Thermal, Potential, Strain, Light, Chemical

3. Machines turn one type of energy into another. Give an example and say what energy changes take place. Electric motor changes electrical into kinetic

4. When energy changes state, some of it is usually made unavailable by what force? Friction

5. This force normally changes moving energy into what? Heat 6. If half the energy is made unavailable we say that the machine is _____% efficient?

Only 50% efficient 7. Plants convert what sort of energy into what when they do photosynthesis?

Light into chemical

8. If the wood is burnt, this energy is transferred into what? Chemical to Thermal

9. If this turns water into steam under pressure, what sort of energy is that? Thermal into kinetic potential (The molecules in the steam are moving faster)

10. If this turns a turbine, what sort of energy is that? Kinetic potential to kinetic

11. If the turbine turns a generator, what sort of energy is produced? Electrical

12. If this lights a light bulb, what sort of energy is produced? Light and thermal

13. Is this an efficient way of giving yourself light?

25

Convenient yes but a lot of energy is ‘lost’ during the process

14. If we continue to use energy at the current rate it is estimated that coal and oil will become too expensive within the next 20 years, why is this? We are using the reserves and no more is being made so the reserves will become less available and therefore more expensive.

15. What alternative sources of energy could we be using? Give several examples. Solar, wind, wave, geothermal, tide

16. Energy is measured in what unit? Joules Units of Measurement

Quantity being measured Name of unit Symbol

length metre m

mass kilogram kg

time second s

force newton N

temperature degrees celsius °C

speed metres per second m/s

area square metres m2

volume cubic metres m3

Table 2 Standard prefixes

Prefix Meaning Example

kilo 1000 1 kilogram (kg) = 1000 grams

centi 1/100 100 centimetres (cm) = 1 metre

milli 1/1000 1000 milligrams (mg) = 1 gram

micro 1/1 000 000 (1 millionth) 1 000 000 micrometres (µm) = 1 metre

26

There are still some units that do not fit the standard pattern. These are shown in Table 3.

Quantity Standard unit

Other units still used

time seconds minutes and hours

60 seconds = 1 minute

60 minutes = 1 hour

length metres miles

(most distances on road signs are still given in miles)

speed m/s kilometres per hour (km/h)

miles per hour (mph)

volume litres 1 litre = 1000 cm3

1 ml = 1 cm3 (1 millilitre is the same volume as 1 centimetre cubed)

Forces

1. Friction is a force that stops movement. Give an example of useful friction. Grip on the soles of your shoes

2. Give an example of friction being reduced on purpose.

Oiling the bearings on your bike, waxing skies

3. How might you measure the friction of your shoe on a carpet? By seeing how many Newtons it takes to pull it along with a spring balance

4. Forces are measured in Newtons. Why? Because Isaac Newton discovered it.

Mass and weight The mass of something is the amount of substance or ‘matter’ it contains. It is measured in kilograms (kg). Weight is the force of gravity pulling on a mass. It is a force, so it is measured in newtons (N). Gravity Gravity is the force of attraction between two masses. The force of gravity is stronger if: • the objects have large masses

• the objects are close together.

On Earth, the gravity pulls on every kilogram of mass with a force of 10 N.

27

5. Gravity is a force that holds the solar system together, and you onto the Earth. What two equations do you need to know to change your masses (g) into weights (N). 100g = 1N 1Kg = 10N

6. What is your mass? Pupil 40kg Adult 90 Kg

What is your weight? Pupil 400N Adult 900N

7. What is the mass of a pencil case? About 300g What is its weight? 3N

8. When unbalanced forces are applied to an object it will do any of three things, what are they? Change shape, change direction, change speed

Balanced forces

The upwards and downwards forces on this balloon are balanced. The balloon will not move.

The forces here are balanced. The girl will not move, and neither will the wall!

9. If a car is travelling at 30 mph, what can you say about the forces? The pushing forces are equal to the friction forces

10. If the car then brakes, what can you say about the forces? The friction force gets greater than the pushing force

11. If the car them stops, what can you say about the forces?

The friction force is the same as the push force both are 0N

12. What force works against gravity when a parachutist comes down? Air resistance

13. What force works against gravity when a boat floats? Upthrust

14. When a force of 3N is applied to a spring it stretches 5 cm, how much will it stretch if 9

N are applied. 3Nx3=9N so 5cmx3 = 15cm

28

15. Why can you not say how much it would stretch for 20N Because you do not know if the spring has overstretched

16. How much would two springs connected in parallel stretch for 3N

3N = 5cm so with two springs the stretch is halved ie 2.5cm

17. How much would two springs in series stretch for 3N In series both springs carry the force so each will stretch 5cm ie 10cm

18. Draw a rough sketch of a graph which summarises the previous 4 questions. Label the axes.

Series Stretch Single Parallel

Force in Newtons

Levers Forces can be used to turn objects around pivots. A pivot is also known as a fulcrum. Levers work by magnifying the force that is put in or the distance it moves.

Moments A turning force is called a moment. Moments are measured in newton centimetres (N cm) or newton metres (N m).

Small moment. Big moment.

29

The longer the distance the greater the moment. It is easier to turn the long spanner than the short one. When an object is balanced, the anticlockwise moment = the clockwise moment.

In the example above: the anticlockwise moment = the clockwise moment 300 N x 2 m = 400N x 1.5 m 600 Nm = 600 Nm The clockwise and anticlockwise moments are the same, so the seesaw is balanced or in equilibrium. Cranes use the principle of moments. The moment from the load is balanced by the moment from the concrete blocks to stop the crane toppling over.

20. Draw a diagram of a lever balanced. 4Kg 3Kg 3m 4m 21. How do you work out the clockwise moments? 3kg x 4m = 12 22. How do you work out the anticlockwise moments? 4Kg x 3m = 12 23. How could you balance Fred who is 60Kg and is sitting 3 m from the

pivot with Gertrude who is only 30Kg F x D = F x D 60 x 3 = 30 x D 120 ÷ 30 = D

4 = D Ans = 4m

30

Speed/Distance/Time Speed tells us how fast something is going. We can work out the mean (average) speed of something by using this formula: speed = distance travelled ÷ time taken. Speed can be measured in: • metres per second (m/s)

• kilometres per hour (km/h)

• miles per hour (mph).

Forces Balanced forces are forces which are the same size but work in opposite directions. Unbalanced forces make things change speed, change shape or change direction. If forces are balanced: • a stationary object stays stationary

• a moving object continues to move at the same speed.

If forces are unbalanced: • a stationary object will start to move

• a moving object will change its speed or direction.

The motorbike is accelerating because the forward force is greater than the backward force.

The motorbike is going at a steady speed. The forces are balanced.

A car or motorbike uses fuel to move at a steady speed because it needs a force from the engine to balance the forces of air resistance and friction.

The amount of air resistance on something can be reduced by giving it a smooth, streamlined shape.

Air resistance is caused by air particles hitting the moving object.

31

The particles transfer energy to the object, which is why objects moving through air can get hot.

The forces on a skydiver change during a jump. Her weight is the same all the time, but her air resistance changes during the jump. We can use a speed–time graph to show what happens.

• At A she has just jumped out of the plane so she has only just started to move downwards. Her air resistance is very small.

• At B her air resistance is bigger, but not as big as her weight so she is still gaining speed.

• At C the forces on her are balanced so she falls at a steady speed.

• At D she has opened her parachute. The air resistance force is suddenly a lot bigger than her weight, so she slows down.

• At E the forces are balanced again, and she will continue to fall at a steady speed until she reaches the ground.