# P5a: Satellites, Gravity and Circular Motion

Satellites have played a major part in the global communications revolution. We can call someone on the other side of the world using a mobile phone or watch events around the world, as they happen, in the comfort of our own homes. This item looks at what satellites are, their uses,including communications and satellite TV, and the physics behind what keeps them in the correct orbit. Newton’s experiment illustrates how uncertainties about science ideas change over time, and the use of models to explain phenomena.

What is gravity?
Gravity is the universal force of attraction between masses.
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What is a satellite?
A satellite is an object that orbits a larger object in space.
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Describe the difference between artificial and natural satellites.
Artificial satellites have been built by people and launched into orbit using rockets.
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Describe how the height above the Earth's surface affects the orbit of an artificial satellite.
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).
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How does the height of an artificial satellite determine its use?
Low orbit polar satellites have uses such as monitoring the weather, observing the Earth’s surface and military uses including spying. Geostationary satellites have uses such as communications, including satellite TV, global positioning/GPS - sat nav
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Describe the conditions of low orbit polar satellites.
They pass over the poles and 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.
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Describe the conditions of geostationary satellites.
Have a different trajectory to polar satellites, they are in orbit above equator. The height of their orbit is 36,000 km, right distance so it takes them 24 hours to make each orbit. This means they stay in a fixed position over Earth's surface
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Name six uses of artificial satellites.
Communications, weather forecasting, military uses, scientific research, GPS and imaging the Earth.
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Explain why the Moon remains in orbit around the Earth - and the Earth and other planets orbit the sun.
Gravity is a force of attraction between masses. It provides the centripetal force needed to keep a satellite in orbit around a planet, or a planet in orbit around a star such as the Sun.
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Describe the orbit of a geostationary artificial satellite.
Orbits the Earth once in 24 hours around the equator, remains in a fixed position above the Earth's surface and orbits above the Earth's equator.
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What does circular motion require?
Gravity which provides the centripetal force for orbital motion.
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Why do different satellites require different orbits?
Satellites in lower orbits travel faster than those in higher orbits. The higher the orbit of a satellite, the longer its 'period'. Geostationary satellites have a different trajectory to polar satellites.
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What is trajectory?
The path followed by a moving object.
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Describe the variation of gravitational force with 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. Force = 1/d^2
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Why don't satellite television dishes need to move?
Because Geostationary satellites always appear in the same position when seen from the ground.
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Explain the variation in speed of a periodic comet during its orbit around the sun.
A periodic comet has a highly elliptical orbit. When a comet comes close to Sun, the gravitational force attraction is high & travels quickly. When comet travels far from Sun, the gravitational force of attraction is less & travels more slowly.
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Explain how the orbital period of a planet depends upon its distance from the Sun.
The further away a planet is, the further it has to travel in one orbit, and the weaker the gravitational force of attraction between it and the Sun.
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Explain why artificial satellites in lower orbits travel faster than those in higher orbits.
sat in lp orbit is close 2 ground,so gravit attraction's strong.this creates high centripetal acceler,so tangential speed's high.geo sat is further,so gravit attraction's weaker.this produces a lower centripetal acceler so the tangential speed's less
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Why are artificial satellites are continually accelerating towards the Earth?
he 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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What happens if the tangential motion is too slow?
if the tangential motion is too slow, the satellite will fall to Earth.
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What happens if the tangential motion is too fast?
if the tangential motion is too fast, the satellite will travel away into space
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## Other cards in this set

### Card 2

#### Front

What is a satellite?

#### Back

A satellite is an object that orbits a larger object in space.

### Card 3

#### Front

Describe the difference between artificial and natural satellites.

### Card 4

#### Front

Describe how the height above the Earth's surface affects the orbit of an artificial satellite.

### Card 5

#### Front

How does the height of an artificial satellite determine its use?