Planetary orbiting

Objects orbit objects larger than themselves. For instance:

• – The Moon orbits the Earth in a circular orbit.
• – The Earth and the other planets in this solar system orbit the Sun in an elliptical orbit.

There’s a centripetal force present due to the gravitational forces of attraction between the larger object and the smaller ones.

In order for the planets to remain in orbit at their specific distances they need to travel at a particular speed.

• – If they travel too fast then they’ll spiral out, away from the Sun.
• – If they travel too slowly they’ll spiral into the Sun.

The planets further from the Sun travel at a slower speed because the gravitational force is weaker. This means that they take longer to orbit the Sun than those planets closer to the Sun.

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Gravitational forces

All object exert a gravitational force on each other. Gravity was discovered by Isaac Newton. He deduced that between any two objects gravity:

• – is a force of attraction
• – increases as the mass of the objects increases
• – decreases as the distance between the objects increases

For example, imagine a space probe being sent from Earth towards the moon. As it takes off, the force of gravity acting on it from Earth decreases. As it moves closer to the Moon the force of gravity acting on it from the Moon increases.

On Earth the gravitational field on the surface us 10 N/kg. On the Moon the gravitational field on the surface is 1.6 N/kg. This is because the Earth has a greater mass to the Moon and so exerts a greater force.