Astronomy

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The Solar System:

Orbits of planets - elliptical (squashed circles) and Sun is close to centre

Gravitational field strength - strength of gravity on planet or moon

Planets don’t emit light - non-luminous

Planets reflect light from Sun

Planets held in orbit by gravitational pull of Sun - stronger the closer they are to Sun

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Moons:

Natural objects which orbit planet

Motions determined by gravitational forces

Non-Luminous, reflect light from sun

Takes moon 29.5 days or 1 lunar month to orbit Earth

Spins on axis like the Earth, but more slowly, completes one rotation every lunar month

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Comets:

Orbit the Sun

1-30km in diameter

Made of dust and ice

Orbits are elongated - can be very close to Sun or at outer reaches of Solar System

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Asteroids:

Minor planets or rocks orbiting Sun

Belt of asteroids between orbits of Mars and Jupiter

Few m to 100s of km

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Satellites:

Objects which orbit a planet

Held in orbit by gravitational forces

Moons - natural satellites

Manufactured objects which orbit planet are artificial satellites

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Gravitational Forces:

Movements of all astronomical bodies determined by gravitational forces.

Newton’s Laws of Gravity:

Between any two objects, force of attraction due to masses of objects

The force is gravitational force

Size of force depends on:

Masses of two objects

Distance between masses

Moving in a Circle:

Planets travel in orbits around Sun

Force has to be applied on them

Force is gravitational attraction between two masses

Gravitational attraction between mass of Sun and each of planets which holds Solar System together and causes planets to follow curved paths

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Orbital Speeds of Satellites:

Orbital Speed = (2 x π x orbital radius) / time for one complete orbit (T)

Distance - metre

Time - seconds