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Unit 12 EXPLORING THE

SOLAR SYSTEM

Fig. 1 Curiosity , a man-made vehicle,

on the surface of Mars.

How do humans explore ourSolar System?

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Getting started

ground stationtransmittingthe TV signal

satellitereciever dishon a house

microwavesignal

relay to

satellite

relay toconsumer

Earth

satellitein orbit

Fig. 2 A satellite orbiting

the Earth.

What do we use satellites for in our everyday lives?

acceleration

air resistance and friction

Fig. 3The arrows show

the forces acting

on a car.

Place a tick in the correct answer box for each question.

• When a car travels at a constant speed:

There are no forces acting on it [ ]

The forces acting are balanced [ ]

The forces acting are unbalanced [ ]

• When the car accelerates:

There are no forces acting on it [ ]The forces acting are balanced [ ]


• When the car decelerates:

There are no forces acting on it [ ]

The forces acting are balanced [ ]


Satellite

An object

that orbits aplanet, suchas the Earth,or anotherlarge objectin space.Satellites maybe natural orman-made.

Accelerate

This means toget faster.

Decelerate

This means toget slower.

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ConceptsIn completing this unit you will learn to:

• Explain the effect of gravity in the

Solar System (E5)• Compare advantages and

disadvantages of different methodsof space exploration (E6)

Key TermsThe meanings of these terms can be foundin the glossary on pages 275–90.

accelerate

astronaut

decelerate

electromagneticradiation

gravitational field

gravitational fieldstrength

gravity

lenses

mass

Moon

Newton

orbit

probe

satellite

shuttle

Soviet Union

space race

telescope

weight

Investigating Scientifically

S8

Success CriteriaLearning outcome E5

Here is what you might aim to achieve by the end of this unit:

• Emerging – outline the movement ofbodies in the Solar System

• Developing – describe the effect of

gravity in our Solar System

• Mastery – explain the effect of

gravity in the Solar System

What level do you think you will be able to achieve?

Learning outcome E6

Here is what you might aim to achieve by the end of this unit:

• Emerging – identify ways in whichhumans learn more about space

• Developing – describe one way in

which humans learn more about

space

• Mastery – compare advantages and

disadvantages of different methods

of space exploration

What level do you think you will be able to achieve?

Learning Outcomes

I know what

these words

mean

A force on an object can change the object’s speed,direction or shape.

Contact forces are forces between objects that are touching.

Non-contact forces are forces between objects that arenot touching.

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1 Why does gravity keep you on the ground?

We know that the Earth’s gravity keeps you on the ground.You feel the Earth’s gravity when you jump off a wall andland! The Earth’s gravity stops you floating off into space.

Fig. 4 IsaacNewton sitting under

an apple tree, just

before the apple fell.

Isaac Newton lived in England in the 1600s. According toan old story, he was sitting beneath an apple tree when hestarted to think about how apples fall.

Before the apple falls, what is its speed?

When the apple is falling, is its speed still zero?

yes [ ] no [ ]

The speed of the apple must be changing when the applefalls. You know that to change an object’s speed, unbalancedforces must be acting on it.

E5

S8

Gravity

This is thecause ofthe forcethat pullseverydayobjects (andus) towardsthe ground.In fact, itpulls ustowards the

center of theEarth.

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weight

supportfrom tree

weight

Fig. 5 When the apple hangs on the tree, theforces are balanced.

Fig. 6 Without support from the tree, theforces are unbalanced. The apple accelerates

downwards.

The Earth’s gravity acts on everything

Everything on Earth (people, cars, houses, planes, clouds,mountains, the atmosphere) is pulled downwards towards thecenter of the Earth. In fact, everything near the Earth is pulledtowards the center of the Earth. This includes communicationsatellites and the Moon.

Moon

Earth

Fig. 7 Everything ispulled towards the

center of the Earth.

The Earth causes a gravitational field. Everything in Earth’sgravitational field has a force acting on it due to the Earth’sgravity.

Gravitationalfield

An areaaround anobject whereits gravitycauses a force.

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9. Using your graph from Activity 1.1, what would be theforce on each of these masses?

0.1 kg

0.15 kg

0.57 kg

1 kg

100 kg

What is the relationship between the mass and the force thatacts on it?

We can calculate the force on an object from thegravitational field strength ( g) and the object’s mass (m).

The force on an object due to the gravitational field is calledthe weight (w) of the object.

On the surface of the Earth, g ≈ 10 N/kg.

But g is not the same everywhere. Study and complete Table 1.

Weight of…

Place Gravitationalfield strength

(N/kg)

1 kg 0.5 kg 10 kg 80 kg

The surface of the Earth 10 10 N

The surface of the Moon 1.6 0.8 N

The surface of Mars 3.8

The surface of the Sun 274 2750 N

The surface of Jupiter 24.9

Table 1

People have walked on the Moon. Your teacher will show you

a video of this.Can you say how people move differently on the Moon? Canyou explain it?

x-axis

The horizontalaxis on a graph.

y-axis

The vertical axison a graph.

Gravitationalfield strength

The force thatis exerted on

every kilogramof an objectthat is in thefield. It isnormally giventhe letter g. Itis measuredin Newtonsper kilogram,N/kg.

Weight

The forceacting on anobject in agravitationalfield, such asthe Earth’sgravitationalfield. Itdepends on the

strength of thegravitationalfield ( g) andthe mass of theobject (m).

w = m g.

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Looking at the information in this section, what problem doyou think there would be for people visiting Jupiter?

Now look at the graph in Fig. 8.

5000 10000 15000 20000 25000 30000 35000 40000 45000 500000

1

2

3

4

5

6

7

8

9

10

11

distance from the Earth’s surface (km)

g (N/kg)

What is the weight of 1 kg at these distances from the surfaceof the Earth? Give your answer in Newtons (N).

0 m

1000 km

10 000 km

40 000 km

50 000 km

The strength of the gravitational pull between two objectsdepends on:

• the mass of the objects

• the distance between the objects.

Fig. 8 This graph

shows Earth’sgravitational field.

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2 How do satellites stay ‘up’?What happens when you throw a ball straight up inthe air?

What happens when you throw a ball out in front of you?

Fig. 9 The ballmoves in the

direction that you

throw it. But at the

same time, it falls

towards the ground

due to Earth’s gravity.

Imagine you could throw a ball very far. Imagine you couldthrow it so far that our picture has to show the curve of theEarth. Fig. 10 shows this.

Fig. 10 You can see

that the ball travelsfurther than it would if

Earth was flat.

Now imagine you could throw the ball very fast. Wheneverthe ball falls towards Earth, due to Earth’s gravity, the Earthcurves away from the ball. This is what happens when asatellite orbits the Earth.

E5

Orbit

The circularor elliptical

path of aplanet aroundthe Sun, orof a moonor artificialsatellitearound aplanet.

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the Earth’s gravitypulls on the satellite

Fig. 11 A satelliteorbits the Earth

but does not fall

towards it because

the satellite is goingvery fast.

Activity 2.1

What to do:

You will need to do this activity outside!

1. Tie the string tightly around the middle of the dowel. Usesticky tape or glue to make sure the knot doesn’t slip.

2. Cut a small slit in the tennis ball, and push the dowel intothe ball.

3. Hold the string tight, about halfway along, and spin it.When it is spinning, try to keep your hand quite still. Moveit just enough to keep the ball spinning.

What is the shape of the ball’s path?

4. Spin the ball with the string longer.

5. Spin the ball with the string shorter.

Equipment:

Some string(about 1 meter),an (old) tennisball, a length of

dowel (5 cm), glueor sticky tape, asharp knife

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What do you notice about spinning the ball when the string isshort, and when the string is long?

Fig. 12 shows the diagram of the spinning ball.

V

F

Fig. 12 The force F on the

ball from the string

keeps it moving in a

circle at velocity V .

When the ball goes

faster, you need

more force to keep

it in the circle. If the

ball goes too slowly,

the circle collapses.

The force on a satellite and the satellite’s speed keep it in

orbit. What force pulls the satellite towards the center?

The force on the satellite is constant. What would happento the satellite if it was moving more slowly? What wouldhappen if it was moving more quickly?

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3 The Solar System

Mars(2 moons)

Earth(1 moon)

VenusMercury

Fig. 13 The Solar System is the Sun and everything that orbits the Sun. Youlearned about orbits in the last section. All planets and moons in the Solar System

are in orbit.

The strongest gravitational force acting on the planets isfrom the Sun, so the planets orbit the Sun. The strongestgravitational force acting on each moon is its nearest planet,so the moons orbit the planets.

E5

Moon

A moon isa naturalsatellite ofa planet.In English,

we use thesame wordas the nameof our moon.Because it’s aname, we giveit a capitalM – the Moon!

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You can see that in Fig. 13 the distances are not to scale. Also,the planets do not really line up like this!

Think about what you have learned about gravity. Explainhow you know that the Sun must have a much larger massthan the planets.

4 Exploring the Solar SystemPeople have known about some planets sinceancient times. But Neptune and Uranus were onlydiscovered in the 1800s. The moons of some planets are stillbeing found today.

Over the last 20 years, scientists have found planets outsideour Solar System. These are planets orbiting distant stars.

E6

Fig. 14 Arab

astronomers at workin ancient times.

Reflection

Why do you think someplanets were discoveredlong ago, and somemuch more recently?What do you thinkpeople used to discoverthese planets?

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Look at the names of the planets in English and in Arabic.The names of the ‘older’ planets (those first discovered) arevery different in the two languages, because different peoplediscovered them in different parts of the world and gave themdifferent names. The ‘newer’ planets were discovered when

news traveled much more quickly.Because Muslim astronomers named many of the stars, theEnglish names for most stars are taken from the Arabic.

An important invention for discovering objects far from theEarth is the telescope.

a)

c)

b)

d)

Fig. 15 Four telescopes: a) Early optical, b) Modern optical, c) Radio telescope,d) The Hubble Space telescope.

The telescope most people think of is the optical telescope,which uses lenses to magnify light. Astronomers also useradio and microwave telescopes to look for objects sending outradiation that is not visible.

Telescopes let us view the Solar System without ever leavingthe planet. But people have always wanted to see further.

Telescope

A devicethat allowsus to look atdistant objectsin detail.

Lenses

Curved piecesof glass used

to focus orenlarge animage, forexample in amagnifyingglass.

Fig. 16 A magnifying glass

has a lens.

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In the 1960s and 70s, the United States and the Soviet Union competed in the space race. Each country wanted to be thefirst to achieve important goals.

Soviet Union

A countrythat wasmade up

of Russia,Ukraine andseveral othermoderncountries.

First man-madesatellite, 1957

Sputnik was the

world’s first artificial

satellite.

First man in space, 1961 Yuri Gagarin

became the first human to visit space.

First woman in space, 1963 Valentina

Tereshkova flew into space in Vostok 6 .

John Glenn in 1962became the first human

to orbit the Earth.

1957 1961 1962 1963

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First shuttle, 1981 Columbia flew into spaceand orbited Earth before returning safely to the

ground.

Fig. 17 A timeline showing

the space race.

The spacerace

Thecompetition

between theUnited Statesand theSoviet Unionto be the firstto achievegoals in spaceexploration.

First probe on the Moon, 1966 Luna 9 landed on the Moon and sent

back photographs from the surface.

First men on the Moon, 1969 Apollo 11 NeilArmstrong and Buzz Aldrin walked on the Moon.

1966 1969 1981

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Travelling in space

Travelling in space is very expensive. A rocket launched intospace needs to work against the Earth’s gravity, and needsenormous amounts of fuel. When people are on board, lots ofmoney must be spent on safety. About 50 years ago, only the

United States and the Soviet Union could afford to have thesevery ambitious space programs.

Space travel is dangerous. Only a few hundred people havebeen into space, and at least 35 have died, normally duringthe launch.

Space is very cold but on the return to Earth a spaceshipbecomes very hot as it enters the Earth’s atmosphere. Thisis a dangerous time for astronauts and spacecraft need towithstand high temperatures.

The Earth’s atmosphere protects people on Earth from dangerous

electromagnetic radiation from the Sun. The spaceship needsto protect astronauts from this radiation and it may limit howlong we can spend beyond the Earth’s atmosphere.

Far from the Earth, the Earth’s gravity is quite weak. Thismeans that our muscles don’t need to work as hard and theybecome weak over time. For trips far away from Earth, forexample to other planets, astronauts would need to do lotsof special exercises to keep their muscles healthy.

On one-time spaceships, for example in the Apollo mission,the outside of the spaceship was allowed to burn up. But

for the space shuttle, it was important not to damage thespacecraft because it will be used for another mission.

Fig. 18 The shuttle was covered with insulating tiles to protect the spacecraftand the crew inside. This silicon tile has been heated to 2200 ˚C, but it is still safe to

touch because it conducts heat very badly.

Astronaut

A person whotravels intospace.

Electro-magneticradiation

Energy thatis given outand travels instraight linesfrom bodies

like the Sun.Light is atype of electro-magneticradiation.Some radia-tion is harmfuland damageshuman cellswhich leads tocancers.

Shuttle

A reusablespacecraftdesigned tosurvive a tripto space andto be usedmany times.Although

the shuttlesthemselveswere reused,they werelifted intospace bya one-userocket.

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The silica glass tiles which cover the space shuttle are verybad conductors of heat. How does this protect the shuttle?

In recent years, more countries have sent probes into space.China, India and the European Union all have active spaceprograms, and the UAE has also launched its first satellite,DubaiSat1.

Manned and unmanned missions

A manned mission is when men and women travel into space.

An unmanned mission uses probes controlled from Earth.Modern probes are like robots. They can react to events.

Which of these missions were manned and which wereunmanned? You may need to use the Internet.

Manned Unmanned

Sputnik [ ] [ ]

Sputnik 2 [ ] [ ]

Luna 9 [ ] [ ]

Manned Unmanned

Apollo 11 [ ] [ ]

Mars 2 [ ] [ ]

DubaiSat1 [ ] [ ]

Fill in this table of advantages and disadvantages:

Mission type Advantages Disadvantages

Telescopes on Earth

Telescopes in space (like Hubble)

Probes to the Moon and the planets

Probes that travel far, and even leave theSolar System (for example Voyager )

Manned mission

Rovers (for example on Mars)

Observation from Earth

Table 2

Probe

An unmannedcraft, oftensmall, which is

sent into spaceor to otherplanets, moonsor cometsfor scientificresearch. Wealso sendprobes deepinto the ocean.

RoverA spaceexplorationvehicledesigned tomove acrossthe surface ofa planet.

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5 Presentation Task You have seen some of the discoveries we have made in space.The USA alone has spent over $900 000 000 000 on exploringspace.

With your team, research the reasons for and against mannedspace travel. Make sure you consider both sides of the discussion.

Your team task is to produce a presentation giving details ofwhy you support or oppose manned space exploration.

Think about these questions:

• What is the purpose of exploration?

• How much does a mission cost?

• What are the limits to what probes and rovers can do?

• What are the limits to what humans can do?

• What science do we learn from the missions themselves?• What other benefits are there from space missions?

• What else could we spend the money on?

Use the Internet and the library to research your answers.

You could present your work in one of these ways:• A poster

• A classroom display

When you are ready, your group will present your argumentsfor or against. You will listen to other groups who are arguing

the other way. Be ready to ask and answer questions!

6 Feedback Medals and Missions

Self Assessment

Shade in the level you have achieved for each outcome in this unit.

Concept LearningOutcome

Emerging Developing Mastery

E5Outline the movementof bodies in the SolarSystem.

Describe the effect ofgravity in our SolarSystem.

Explain the effect ofgravity in the SolarSystem.

E6Identify ways in whichhumans learn moreabout space.

Describe one way inwhich humans learnmore about space.

Compare advantagesand disadvantages ofdifferent methods ofspace exploration.

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Skill LearningOutcome

Emerging Developing Mastery

S8

Record observations/data in some kind of

systematic way.

Record observations/data in a simple table.

Construct anappropriate and

complete table to recordobservations/data.

Medals

What have been your greatest achievements during this unit? For example,mastering a concept outcome, improving a skill or feeling proud of yourorganizational abilities, team work or presentation.

What did you do well? How did you do it?

1.

2.

3.

MissionsWhat are your targets for improvement? Select two Learning Outcomes to focuson and set yourself a target. For example, if you have reached ‘developing’, whatdo you need to do next time to achieve ‘mastery’?

Learning Outcomes Target

1.

2.

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7 Science I have learned in this unit

● Mass is constant. The weight of an object depends on its ownmass, and the mass of the object that gravity pulls it towards.

● Objects in the Solar System move in orbits.

● Orbital movement is due to gravity.

● Smaller objects orbit larger objects.

●

Humans can explore space from the Earth by travelling intospace, or by sending unmanned probes.

● Telescopes are useful for exploring space from the Earth.

● Manned missions are the most expensive way to explore space.

● Some telescopes, like the Hubble, are situated in space.

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