Transcript Gravity and space File

9J
Gravity and space
What is gravity and
why is it so important?
Exploring Science 9
Pearson Education Limited 2004
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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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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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9Ja
Gravity
All objects exert a small force on other objects
which tries to pull them together. The force is
called gravity.
The bigger the mass of an object, the greater
its gravitational pull.
The Earth has an enormous
mass. Its gravity is very and
acts downwards, towards the
centre of the Earth.
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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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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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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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Try these…
Unscramble these letters, and then
explain what the word means.
• tivyarg
• __av___
• gravity
• An attractive force possessed by all
objects that have mass.
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Unscramble these letters, and then
explain what the word means.
• gewhit
• __ig__
• weight
• The force of gravity acting on the mass
of an object.
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Unscramble these letters, and then
explain what the word means.
• wennot
• __wt__
• newton
• The unit of weight, named after Sir
Isaac Newton.
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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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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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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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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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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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True or false?
2. The weight of an object depends on its
mass and the force of gravity.
True
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True or false?
3. You weigh less at the top of a
mountain than you do at sea level.
True
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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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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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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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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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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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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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An elliptical orbit
planet
The planet is moving
fastest when it is at this
part of its orbit.
Sun
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The planet is moving
slowest when it is at
this part of its orbit.
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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.
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A Aristotle
B Kepler
C Egyptians
D Ellipse
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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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9Jd
Gravity and the planets
The Sun’s gravity keeps the Earth and the
other planets moving around it in elliptical
orbits.
The Earth’s gravity keeps the Moon in its
orbit.
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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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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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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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 36 000 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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Different orbits
A satellite in a
polar orbit covers
all parts of the
Earth as it spins.
Observation
satellites have
polar orbits.
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path of satellite
on its last orbit
path of satellite
this orbit
path of satellite
on its next orbit
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9Jd
Try these…
Question: What type of orbit does the
Earth have around the Sun?
or
What type of orbit is oval?
Answer:
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Elliptical
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Question: What keeps the Earth from
flying off into space?
or
What force exists everywhere
in the Universe?
or
What holds galaxies together?
Answer:
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Gravity
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Question: What do we call anything that
orbits a planet?
or
What is the Moon?
Answer:
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A satellite
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Question: Which satellites can be used
for weather forecasting?
or
Which satellites follow polar
orbits?
Answer: Earth observation satellites
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How much do you know?
The force which tries to pull objects
together.
G______
Gravity
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How much do you know?
Weight is measured in these units.
N______
Newtons
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How much do you know?
Its gravity is about one-sixth of the
Earth’s.
M___
Moon
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How much do you know?
A Greek philosopher who thought the
planets and the Sun circled the Earth.
A________
Aristotle
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How much do you know?
He first suggested our modern model
of the Solar System.
K_____
Kepler
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How much do you know?
The type of orbit followed by the
planets around the Sun.
E_________
Elliptical
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How much do you know?
The Moon is an example.
N______ s________
Natural satellite
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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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