Astronomy

PMM 2012

End of 3rd Form

• At the end of last school year many of you covered the topic of Astronomy.

• During the next few lessons I am going to check you fully understood the concepts before moving on.

• You will have a test on Chapter 6 next week.• You will receive a revision handout this lesson.

Key words- quick test• Star• Planet• Luminous/non-luminous• Orbit• Ellipse• Solar system• Comet• Asteroid• Moon• Satellite (Artificial/non-artificial)• Galaxy• Milky way galaxy• Galaxy cluster

• Gravity• Weight• Period T• Circular motion• Centripetal force

Distances in space• How far is 1km?• 100km (travel for an hour by car or to the edge of the

atmosphere straight up)• Distance to Moon 400 000km away• Distance Sun to Neptune 4,350,000,000km• Distance to nearest star 39,900,000,000,000 km or 4.35 light

years• Distance to centre of galaxy 100,000 light yearsThe distance to

the galaxy M87 in the Virgo cluster is 50 million light years. • The distance to most distant object seen in the universe is

about 18 billion light years (18 x 109 light years).

GravityAny object that has mass has gravityAny object that has mass has a gravitational field around it which can be pictured as

The lines show the direction of the force of gravityThe size of the gravitational field is given by the symbol ‘g’The size of ‘g’ depends on the mass of the planet and how close you are to the planet

Weight

• By Weight=mass x g W=mg• W in Newtons Mass in kg g in N/kg• ‘g’ varies on other planets e.g. it is 1.67N/kg

on the Moon• ‘g’ a long way from any planets is zero

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Star• Stars begin as a huge cloud of dust, ice and gas

which are brought together by gravity until they are hot enough for fusion to begin. They then give off huge amounts of energy.

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The Sun is a star

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Planet

The heat of the Sun pushed some matter outwards which formed the planets of the solar system.

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Solar system

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Solar system

• All the planets orbit the Sun• All the planets orbit the Sun in the same

direction• All the planets take a different time to orbit the

Sun. This time is called the PERIOD. Symbol T• The planets were formed at the same time as

the Sun• The Sun and planets were made from a cloud

which was left over when a star exploded.

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Size of planets

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Luminous/non-luminous

The Sun and other stars give off light due to fusion. They are called luminous.

All other objects in the Universe do not give off their own light. We see them by reflection only and they are non-luminous.

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Ellipse

Wait on this slide!

The Earth is at one focus of the Sun!

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Planets in ellipses

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Comet

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Comet details

• Comets orbit the Sun• 1-30km across• Made of dust and ice• Very elliptical• Tail of comet always points away from the Sun• Tail forms when ice melts when the comet is

nearest the Sun

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Comet tails

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Asteroid

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Asteroid details

• Minor planets or rocks that orbit the Sun• Belt between Mars and Jupiter• Formed when Solar system formed• May have been a planet that then broke up

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Moon

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Satellite

Natural satellites are moonsArtificial satellites are put into

space and have many uses e.g. communications, weather, spying, geological surveys

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Types of orbit

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Galaxy

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Milky Way Galaxy

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Milky Way from Earth

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Galaxy Clusters

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Structure of the Solar system

We have already looked at the structure of the Solar system and the orbits of

• Planets around the Sun• Moon around planets• Comets and Asteroids around the Sun• Satellites around the Earth

Question: How do they stay in orbit?

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Circular motion

Don’t try this at home! When the petrol-soaked ball is lit we have to keep swinging it around. Don’t stand too close!

Where will the fireball hit if the juggler lets go at X?

X

Here?

Here?

Here?

Here?

A double-decker roundabout in Paris. Someone sitting on the horses will be constantly accelerating because they are constantly changing direction. What happens if they try to get off while it is moving?

What direction will the passenger travel if they get off the roundabout at X?

X

Here?

Here?

Here?

Here?

Circular MotionKey points

1. Object moving in a circle at constant speed, is changing direction and is therefore changing velocity. We call this acceleration.

2. This is a special acceleration as it is always toward the centre of the circle. It does not cause the object to go any faster, but it causes it to change direction.

3. We call this a centripetal acceleration.4. Centripetal acceleration is caused by a force which we call

centripetal force.5. Both the force and the acceleration are towards the centre

of the circle.

http://web.ics.purdue.edu/~mjcarlso/ST/ST016_Circular_Motion.m4v

mad American!!http://www.youtube.com/watch?

v=L6-kn2tB-9E

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Show video clips in folder f1, f2 f3

Facts to know:

• Each planet has a different orbital speed. The year for each planet gets longer as you go out from the sun

• The speed of each planet is different. A planet must have a particular speed for each orbit. Too slow= spiral in Too fast = fly out of orbit

Period

• Time it takes a satellite to make one orbit of earth.

• Higher orbit, slower speed, travels further so period is longer

• Lower orbit, faster speed, travels less far so period shorter

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The Maths bitHow far is it around the circle?

Answer = 2 x π x radius

How long does it take to complete one orbit? This is called the period and the symbol is T

How fast does it travel?Speed = distance time

Equations

Speed = distance /time LEARN

Distance around the orbit = 2 LEARN

Orbital speed = 2 GIVEN IN THE EXAM T