1. What is gravity and why is it so important?
9J Gravity and space
What is gravity and why is it so important?
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2. 9J
You should know…
9Ja What gravity and weight are and how they are measured.
9Jb How gravity changes depending on where you are.
9Jc How our ideas about the Solar System have changed.
9Jd The importance of gravity for planets, satellites and spacecraft.   
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3. You should know… What causes weight. Which direction ‘down’ is.
9Ja
You should know…
What causes weight.
Which direction ‘down’ is.
How gravity affects objects.
How to work out the weight of an object.
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4. 9Ja
Gravity
All objects exert a small force on other objects which tries to pull them together. The force is called
The bigger the mass of an object, the greater its gravitational pull.
gravity.
The Earth has an enormous mass. Its gravity is very and acts downwards, towards the centre of the Earth.
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5. 9Ja
Mass and weight
Weight and mass are connected, but they are not the same thing.
Mass is the amount of matter in an object.
It is measured in kilograms (kg).
The force of gravity pulling on an object’s mass is its weight.
Weight is measured in
newtons (N).
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6. Mass and weight The force of gravity on Earth is 10 N per kg.
9Ja
Mass and weight
The force of gravity on Earth is 10 N per kg.
This man’s mass is 90 kg, so his weight is 90 x 10 = 900 N.
This boy’s mass is 45 kg, so his weight is 45 x 10 = 450 N.
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7. 9Ja
Sir Isaac Newton
The unit of weight is named after Sir Isaac Newton, who lived from 1642 to 1727, and carried out important work on forces.
Newton worked out how to calculate the force of gravity on objects.
A falling apple is said to have inspired Newton to create his laws of gravity when it fell on his head. It has a weight of about one newton.
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8. 9Ja
Try these…
Unscramble these letters, and then explain what the word means.
t i v y a r g
_ _ a v _ _ _
gravity
An attractive force possessed by all objects that have mass.
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9. Unscramble these letters, and then explain what the word means.
9Ja
Unscramble these letters, and then explain what the word means.
g e w h i t
_ _ i g _ _
weight
The force of gravity acting on the mass of an object.
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10. Unscramble these letters, and then explain what the word means.
9Ja
Unscramble these letters, and then explain what the word means.
w e n n o t
_ _ w t _ _
newton
The unit of weight, named after Sir Isaac Newton.
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11. You should know… Why gravity is different on other planets.
9Jb
You should know…
Why gravity is different on other planets.
How to calculate weight on other planets.
How gravity changes with distance.
How changing gravity affects spacecraft.
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12. Gravity on the Moon The Moon is about one sixth the mass of the Earth.
9Jb
Gravity on the Moon
The Moon is about one sixth the mass of the Earth.
If you went to the Moon, your mass would stay the same but your weight would decrease.
Your mass does not change because you are still made up of the same amount of matter.
Your weight changes because the Moon’s gravitational pull on your body is less than that of Earth.
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13. 9Jb
Gravity on the Moon A B
Twin A on Earth has a mass of 60 kg and a weight of 600 N.
Twin B on the Moon has a mass of 60 kg and a weight of 100 N.
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14. 9Jb
Gravity in space
The force of gravity gets less as objects move further apart.
As a spacecraft gets further away from the Earth, the pull of the Earth’s gravity on it gets less.
At a certain point between the Earth and the Moon, the pull of the Earth is cancelled out by the pull of the Moon.
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15. Try these… True or false?
9Jb
Try these…
True or false?
1. You weigh more on the Moon than on the Earth.
False
You weigh less on the Moon than on the Earth, because the Moon’s gravity is about one sixth that of the Earth’s.
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16. 9Jb
True or false?
2. The weight of an object depends on its mass and the force of gravity.
True
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17. 3. You weigh less at the top of a mountain than you do at sea level.
9Jb
True or false?
3. You weigh less at the top of a mountain than you do at sea level.
True
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18. 9Jb
True or false?
4. On the Moon, your weight is the same but your mass is less than on Earth.
False
Your mass is the same wherever you are, but your weight is less on the Moon than on Earth.
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19. You should know… About some early ideas about the Solar System.
9Jc
You should know…
About some early ideas about the Solar System.
How our modern ideas are different to the early ideas.
Our current model of the Solar System.
Some of the evidence for our current model of the Solar System.
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20. 9Jc
The shape of the Earth
The ancient Egyptians believed that the Earth was a flat plain with the River Nile running through the middle.
The Babylonians thought the Earth was a large hollow mountain floating in water.
Most people have thought of the Earth as a sphere for the last 2000 years.
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21. Ideas about the Solar System
9Jc
Ideas about the Solar System
Aristotle and Ptolemy both had similar ideas about the Solar System.
They thought that the Earth was at the centre and didn’t move, and the Sun and planets circled round it.
Observations about the positions of the planets on different dates didn’t fit these ideas.
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22. Ideas about the Solar System
9Jc
Ideas about the Solar System
Copernicus produced a better model, with the Earth going round the Sun, but it still wasn’t accurate and was very complicated.
Earth
The Earth orbits around this point.
The centre of the Earth’s orbit is moving around this point, which is itself moving around the Sun.
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23. Ideas about the Solar System
9Jc
Ideas about the Solar System
Our modern model of the Solar System was first suggested by Kepler in 1609.
He discovered that the speed of Mars changed as it moved around its orbit.
He suggested that the planets moved in ellipses, not circles.
We still use this model today.
It is used to calculate where the planets are and the journeys of spacecraft.
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24. An elliptical orbit planet
9Jc
An elliptical orbit
planet
The planet is moving fastest when it is at this part of its orbit.
Sun
The planet is moving slowest when it is at this part of its orbit.
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25. Try these… They believed the Earth to be flat. A Aristotle
9Jc
Try these…
They believed the Earth to be flat.
Thought that the Earth did not move.
How the planets move around the Sun.
Suggested the modern model of the Solar System.
A Aristotle
B Kepler
C Egyptians
D Ellipse
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26. You should know… Why planets stay in their orbits.
9Jd
You should know…
Why planets stay in their orbits.
What a satellite is and some uses of artificial satellites.
About different kinds of orbits for artificial satellites, and why they are useful.
How satellites can help us to explore the Solar System.
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27. Gravity and the planets
9Jd
Gravity and the planets
The Sun’s gravity keeps the Earth and the other planets moving around it in
elliptical orbits.
keeps the Moon in its orbit.
The Earth’s gravity
The gravitational attraction between the stars in a galaxy holds the galaxy together.
There is even an attraction between different galaxies.
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28. Satellites Anything that orbits a planet is a satellite.
9Jd
Satellites
Anything that orbits a planet is a
satellite.
The Moon is an example of a natural satellite.
Artificial satellites have been put into Earth’s orbit. They have many uses:
communications satellites transmit TV pictures or phone calls
the Global Positioning System (GPS) is a set of satellites that tells people their exact position on the ground.
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29. scientific research and
9Jd
Satellites
Earth observation satellites take pictures or measurements of the Earth. They can be used for
weather forecasting,
making maps,
scientific research and
spying on other countries.
Satellites can also be put into orbit around other planets.
Space probes have been sent to Venus, Mars, Jupiter and Saturn.
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30. 9Jd
Different orbits
Communications satellites follow a geostationary orbit so they can be at the same point above the Earth at all times.
They are about km above the Earth and have a rotation period equal to the Earth’s rotation on its axis (24 hours).
Their orbit is usually over the Equator.
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31. 9Jd
Different orbits
A satellite in a polar orbit covers all parts of the Earth as it spins.
Observation satellites have polar orbits.
path of satellite on its last orbit
path of satellite this orbit
path of satellite on its next orbit
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32. 9Jd
Try these…
Question:
What type of orbit does the Earth have around the Sun? or
What type of orbit is oval?
Answer: Elliptical
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33. What keeps the Earth from flying off into space? or
9Jd
Question:
What keeps the Earth from flying off into space? or
What force exists everywhere in the Universe?
What holds galaxies together?
Answer: Gravity
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34. What do we call anything that orbits a planet? or What is the Moon?
9Jd
Question:
What do we call anything that orbits a planet? or
What is the Moon?
Answer: A satellite
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35. Which satellites can be used for weather forecasting? or
9Jd
Question:
Which satellites can be used for weather forecasting? or
Which satellites follow polar orbits?
Answer: Earth observation satellites
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36. How much do you know? The force which tries to pull objects together.
Gravity
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37. How much do you know? Weight is measured in these units. N _ _ _ _ _ _9J
How much do you know?
Weight is measured in these units.
N _ _ _ _ _ _
Newtons
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38. How much do you know? Its gravity is about one-sixth of the Earth’s.9J
How much do you know?
Its gravity is about one-sixth of the Earth’s.
M _ _ _
Moon
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39. 9J
How much do you know?
A Greek philosopher who thought the planets and the Sun circled the Earth.
A _ _ _ _ _ _ _ _
Aristotle
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40. 9J
How much do you know?
He first suggested our modern model of the Solar System.
K _ _ _ _ _
Kepler
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41. 9J
How much do you know?
The type of orbit followed by the planets around the Sun.
E _ _ _ _ _ _ _ _ _
Elliptical
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42. How much do you know? The Moon is an example.9J
How much do you know?
The Moon is an example.
N _ _ _ _ _ _ s _ _ _ _ _ _ _ _
Natural satellite
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43. 9J
How much do you know?
Orbit that allows a satellite to remain in the same place above the Earth.
G _ _ _ _ _ _ _ _ _ _ _ _
Geostationary
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