10.1 The solar system and orbits — Practise with help

1. $3.0 \times 10^{8}\,\text{m/s}$.
2. $v = \dfrac{d}{t} = \dfrac{1.2 \times 10^{4}}{4.0} = 3.0 \times 10^{3}\,\text{m/s}$.

If question 1 went wrong, it is worth remembering: light and all other electromagnetic waves travel at $3.0 \times 10^{8}\,\text{m/s}$ in a vacuum. If question 2 went wrong, revisit the speed equation in Chapter 3.

Start with the star at the centre. Then add the bodies that orbit the Sun, and the bodies that orbit the planets.

Write a list with one line for each kind of body: star, planet, dwarf planet, moon, artificial satellite, asteroid and comet.

The solar system has one star, the Sun. Eight planets orbit the Sun, and dwarf planets orbit it too. Moons, which are natural satellites, orbit the planets. Artificial satellites built by people orbit the Earth and other bodies. Asteroids and comets orbit the Sun. The whole solar system is a small part of the Milky Way galaxy.

Ask what each body orbits, and whether it was made by people or by nature.

Write two short sentences. The first says what the two bodies have in common. The second says one clear way in which they are different.

A planet orbits a star, and a moon orbits a planet. They are similar because gravity keeps both of them in orbit. They are different because a planet is large and orbits the Sun, while a moon is smaller and orbits a planet.

Name the force. Then say which way it points.

Say that the force points towards the Sun, the centre of the orbit. Then say what the force does to the direction of the planet.

Gravity provides a force of attraction between the planet and the Sun. The force is directed towards the Sun, which is the centre of the orbit. This force changes the direction of the planet, so its path curves into a circle instead of going in a straight line.

The light travels at a steady speed. Write down the equation that links time, distance and speed.

Use $t = \dfrac{d}{v}$. Put the distance over the speed, keeping the powers of ten together.

Light takes 1.28 s to travel from the Moon to the Earth.

To compare two distances, divide the larger by the smaller.

Divide the distance of Jupiter by the distance of the Earth. The powers of ten are the same, so they cancel.

Jupiter is 5.2 times further from the Sun than the Earth is.

There are two opposing effects. One comes from fusion and one comes from gravity.

Describe what fusion does to the size of the Sun, then describe what gravity does to it. Finish by comparing the sizes of the two effects.

Fusion reactions in the core release energy, which pushes outwards and tends to make the Sun expand. Gravity pulls the material of the Sun inwards and tends to make it collapse. The outward push and the inward pull are equal, so the Sun is in equilibrium and stays the same size.

Start with the cloud of dust and gas. Then follow the order of the stages.

Name four stages in order: the cloud, the main-sequence star, the red giant and the white dwarf.

The star forms from a cloud of dust and gas pulled together by gravity. Fusion starts, and the star becomes a main-sequence star like the Sun. Later the star expands into a red giant. It then cools and collapses into a white dwarf.

Recall what the words speed and velocity each mean. One of them includes a direction.

State that velocity is speed in a given direction. Then describe what happens to the direction as the satellite goes round the circle.

Velocity is speed in a given direction. As the satellite goes round its circular orbit, the direction of its motion changes all the time. Its speed stays the same, but the changing direction means that its velocity changes.

A stable circular orbit is a match between two quantities: the speed and the radius.

Say what a stable orbit needs. Then say what happens if the speed changes while the radius stays the same.

A stable circular orbit needs a speed that matches its radius. If the speed changes, the satellite can no longer stay in a stable orbit at the same radius. The radius must change until the speed and the radius match again, so the orbit becomes stable at the new radius.