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Centripetal force and gravity
A moving object will continue to move in
a straight line at the same speed unless a
force acts on it. For an object to move in a
circle, a force has to act on it all the time.
This force is called the centripetal force. It
acts towards the centre of the circle.
Gravity is the centripetal force that keeps
planets moving around the Sun, and
satellites moving around planets.
The Earth has many artificial satellites in
orbit around it. These have been built by
people and launched into orbit using
rockets. Some very large artificial
satellites were put into orbit by the
American Space Shuttle.
Orbits
Artificial satellites in orbit around the
Earth have different orbits. Satellites in
lower orbits travel faster than those in
higher orbits. The higher the orbit of a
satellite, the longer its 'period' (time to
make one orbit).
Gravity
Gravity is the universal force of attraction
between masses. The greater the mass
involved, the greater the force.
The force of gravity between two objects
with small masses like a table and a chair
is small because the objects are. However,
large objects like planets have a much
greater mass and so have a much greater
gravitational force.
Satellites in low polar orbit pass over the
poles. They orbit between 100 km and
200 km above the Earth’s surface, taking
around 90 minutes to make each orbit.
The Earth spins beneath the satellite as it
moves, so the satellite can scan the whole
surface of the Earth. Low orbit polar
satellites have uses such as:
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Satellites
A satellite is an object that orbits a larger
object in space. The Moon is the Earth’s
natural satellite. Other planets also have
natural satellites. For example, Mars has
two small natural satellites called Phobos
and Deimos.
monitoring the weather
observing the Earth’s surface
military uses including spying
Geostationary satellites have a different
trajectory to polar satellites – they are in
orbit above the equator. The height of
their orbit - 36,000 km - is just the right
distance so that it takes them one day (24
hours) to make each orbit.
This means that they stay in a fixed
position over the Earth’s surface.
Geostationary satellites have uses such
as:

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communications - including satellite
TV
global positioning or GPS - which is
used for sat navs (satellite navigation
systems)
Geostationary satellites always appear in
the same position when seen from the
ground. This is why satellite television
dishes can be bolted into one position
and do not need to move.
Gravity and distance
The force of gravity between two objects
decreases as the objects move further
apart. It follows an inverse square law the force between two objects is inversely
proportional to the distance between
them squared.
Orbit and speed
The orbital period of a planet depends
upon its distance from the Sun. The
further a planet is from the Sun, the
longer its orbital period. This is because
the further away a planet is:

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the further it has to travel in one
orbit
the weaker the gravitational force of
attraction between it and the Sun
Comets
A periodic comet has a highly elliptical
orbit (oval-shaped, rather than circular)
When a comet comes close to the Sun,
the gravitational force of attraction is high
and it travels quickly. When a comet
travels far from the Sun, the gravitational
force of attraction is less and it travels
more slowly.
This can be shown as an equation:
Equation to show force
This means that if the distance between
two objects is doubled, the force between
them drops to a quarter (1/22 or 1/4). If
the distance is increased ten times, the
force between the two objects decreases
to a hundredth of what it was (1/102 or
1/100).
Satellites
Artificial satellites are continually
accelerating towards the Earth. This is
because of the gravitational force of
attraction between the satellite and the
Earth. If a satellite stopped moving
forwards, the Earth’s gravitational pull
would make it fall to Earth.
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The forward speed, or tangential
motion, of a satellite must be just
right to keep the satellite in orbit:

if the tangential motion is too slow,
the satellite will fall to Earth

if the tangential motion is too fast,
the satellite will travel away into
space
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Artificial satellites in lower orbits
travel faster than those in higher
orbits.

A satellite in low polar orbit is close
to the ground, so the gravitational
attraction is strong. This produces a
high centripetal acceleration, so the
tangential speed must be high.

A geostationary satellite is further
from the ground, so the gravitational
attraction is weaker. This produces a
lower centripetal acceleration, so the
tangential speed must be less.
Artificial satellites are continually
accelerating towards the Earth due to the
Earth’s gravitational pull, but their
tangential motion keeps them moving in
an approximately circular orbit