# The Solar System

## The Solar System

#### The Planets

The Solar System is made up of the nine planets, including the Earth, that orbit (go round) the Sun.

See whether you can name them on the diagram below:

The four inner planets are called terrestrial planets and are small rocky planets.

Between Mars and Jupiter, there is an asteroid belt, which may be the remains of a planet.

The next four are called gas giants and are large planets made up of gas.

Pluto is a very cold lump of rock, with a moon - Charon - nearly as large as Pluto.

#### Some facts about the solar system

Decide whether the following facts are true or false:

The moon orbits the earth in 28 days and the Earth orbits the sun in 365¼ days. Each planet takes a different amount of time to orbit the sun.

#### Orbits and circular motion

As mentioned above, the planets move in near circular orbits called ellipses.

Objects will only move in a circle if there is a force acting on them. The force in this case is caused by the gravitational attraction between masses. This is also the force that keeps the moon in orbit round the Earth and your feet on the ground.

But why must there be a force for objects to travel in a circle?

Speed and velocity

Velocity is speed with direction. If a car has driven off at 30mph, it would be very hard to find it after an hour, but if you knew it had driven at 30mph northwards for an hour, it would be very easy to find.

Below is a brief demonstration to show this. You will see three different journeys of the same two cars. For each journey, the blue car drives due north at the same constant speed and so always ends up in the same place. The red car however, drives with constant speed, but with changing direction and so ends up in a different place every time.

If something is going in a circle, its velocity is changing, even if its speed is constant. This is because it is changing direction. If the velocity is changing the object must be accelerating. We know from Newton's Laws that something will only accelerate when a force is being applied.

We call the force that keeps things moving in a circle a centripetal force. A centripetal force can be provided in a number of ways. See whether you can identify which of the forces below provides the force in each situation:

Notice that the centripetal force always acts towards the centre of the circle.

Help the hammer thrower below release the hammer at the right time. Remember, as soon as the centripetal force is removed the object will travel in a straight line.

Satellites

Satellites are also held in orbits around the Earth by the gravitational attraction of the Earth. There are two main types of satellites:

1. Geo-stationary orbit satellites:

The satellite rotates around the Earth once every 24 hours, so it is able to stay over one point on the Earth. This is useful for communications, as microwaves can be directed at the satellite, which then sends them back to a different point on the Earth. The satellite is always in the same position relative to the Earth.

2. Polar orbit satellites:

The satellite can orbit the Earth many times in a day and see the whole Earth as it travels. The Earth is rotating as the satellite orbits, so the satellite sees a different part of the Earth each time it goes around. These satellites are used for weather forecasting and for spying.