Gravity
The force of gravity exists between all masses.
The size of the force depends on:
the masses
the distance between them
Of the two, distance is much more important ­ here's why:
The equation(which you really don't need to know) is:
The force of gravity
F= Gm m
1
2
2
d
The Universal Gravitational constant =
6.67 × 10­11 m3 kg­1 s­2
the two masses multiplied together
divided by the distance between them SQUARED ­ big effect
Gravity is a really, really, really weak force that only becomes really significant with PLANETARY masses
Gravitational Force is directly proportional to mass
F m1
F = G m1.m2
d2
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Gravitational Force is inversely proportional to distance
The force of gravity between two objects:
increases regularly as the mass increases
(directly proportional)
decreases very sharply as the distance increases
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Orbits
These are classic examples of circular motion:
If an object is travelling at 11km/s (7 miles per second) or more then it will escape the earth's gravity ­ This is the escape velocity
Getting into orbit
increasing muzzle velocity
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falling
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...but also moving on
low earth orbit (LEO)
high earth orbit
So, the satellite misses the Earth and falls around it
Higher orbit ­ lower orbital speed
Artificial Satellites
There are two basic types of orbit used:
Polar orbits
These cover the entire globe in slices due to the rotation of the globe and of the satellite:
Their orbits are quite low at around 530 miles/850km.
They are useful as monitoring satellites, and are used by the military, weather forecasters, prospectors and for espionage
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Geostationary/Parking Orbits
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These satellites orbit above the same spot on the equator at 36000km above the ground.
They take exactly 24 hours to complete one orbit.
Used for:
communications satellites
GPS (global positioning systems)
Planetary orbits
The 8, 9 or 10 planets that orbit our sun (called Sol) move in ellipses that are usually (but not always) quite close to being circles ­ yes, I'm looking at you Pluto!
Planetary Orbit Rules
For a planet to orbit at a specific distance from the Sun it must have a specific speed
• planets that are farther from the Sun move slower
• planets that are nearer the Sun move faster
Planetary Orbit Rules
The farther a planet is away from the Sun, the longer it takes to complete one orbit (ie, it has a longer year)
This makes sense:
Lower speed
It must take longer
Greater circumference
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Planetary Orbit Rules
For a planet to orbit at a specific distance from the Sun it must have a specific speed
• planets that are farther from the Sun move slower
• planets that are nearer the Sun move faster
The farther a planet is away from the Sun, the longer it takes to complete one orbit (ie, it has a longer year)
This makes sense: the speed is lower and the circumference of the orbit is greater ­ it must take longer.
What is an ellipse?
The Sun goes on one of the foci of the ellptical orbit, and the planet orbits around the perimeter.
The eccentricity ("ovalness") of the orbits is very small, so the sun is almost at the centre.
In Mathematics, an ellipse (from the Greek ἔλλειψις, literally absence) is a locus of points in a plane such that the sum of the distances to two fixed points is a constant. The two fixed points are called foci (plural of focus).
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Kepler's Laws
A Level
...but I think they're really easy
• First Law
The orbits of the planets are ellipses with the Sun at one focus
empty
The scale is totally bobbins too!
Actually, the orbits are very close to being circular, so the foci are near the centre
You ARE expected to know that the Sun sits on one of the foci, and the planet goes around the perimeter
Kepler's Laws
A Level
• Second Law NEVER been tested
A line joining the planet to the Sun sweeps out equal areas in equal times.
slow
fast
Kepler's Laws
A Level
• Third Law The only scary one
The length of the year, squared, is proportional to the average distance from the Sun, cubed.
Year
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radius
Evidence for Kepler's Third Law
(still A Level)
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Radius 3
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An AU is an Astronomical Unit
which is the average distance from the centre of the Sun to the centre of the Earth
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Year 2
But What About GCSE?
The examiners would like you to be aware that the year length increases as the orbital radius increases
Orbit Radius (AU)
So, the A Level version isn't really all that much more complex!
Smooth curve?
Copy this one
Year length (Years)
An astronomical unit (AU) is the average distance from the Sun to the Earth
Attachments
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