10.1 The solar system and orbits — Practise

Work through these on your own. Keep your working neat: the method matters as much as the answer.

Both the questions and the answers below come from a set that is checked by computer every time the book is built.

State the main features of our solar system.
Give one similarity and one distinction between a moon and an artificial satellite.
Explain what keeps a planet in a circular orbit around the Sun.
Describe how the Sun formed.
Explain why the Sun stays the same size.
Describe the life cycle of a star that has about the same mass as the Sun.
Describe the life cycle of a star much more massive than the Sun. State where the elements heavier than iron are made.
The telescope orbits 540 km above the Earth. A radio signal from the telescope travels at $3.0 \times 10^{8}\,\text{m/s}$ . Say which equation you will use, then calculate the time for the signal to reach the ground. Give your answer to 2 significant figures.
The Sun is $1.5 \times 10^{11}\,\text{m}$ from the Earth. Light travels at $3.0 \times 10^{8}\,\text{m/s}$ . Calculate how long light takes to travel from the Sun to the Earth. Give your answer to 2 significant figures.
The Moon is $3.84 \times 10^{8}\,\text{m}$ from the Earth. Calculate how long light takes to travel from the Moon to the Earth. Give your answer to 3 significant figures.
Jupiter is $7.8 \times 10^{11}\,\text{m}$ from the Sun. The Earth is $1.5 \times 10^{11}\,\text{m}$ from the Sun. Work out how many times further from the Sun Jupiter is than the Earth.
A pupil says that the telescope floats because there is no gravity in space. Explain what is wrong with this statement.
H The telescope moves at a constant speed in a circular orbit, but its velocity changes. Explain how this is possible.
H Explain, for a stable orbit, why the radius must change if the speed of the object changes.

Answers and concise solutions

1. The main features of the solar system

The solar system has one star, the Sun.
Eight planets orbit the Sun, and dwarf planets also orbit the Sun.
Natural satellites, called moons, orbit the planets.
Artificial satellites built by people also orbit the Earth and other bodies.
The solar system is a small part of the Milky Way galaxy.

2. A moon compared with an artificial satellite

Both orbit a larger body, and gravity keeps both of them in orbit.
A moon is natural and always orbits a planet. An artificial satellite is built by people and can orbit a planet or another body.

3. What keeps a planet in its orbit

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

4. How the Sun formed

The Sun formed from a cloud of dust and gas, called a nebula.
Gravity pulled the dust and gas together.
As the cloud was pulled together it became hot and dense enough for hydrogen nuclei to fuse and release energy.

5. Why the Sun is stable

Fusion reactions release energy. This 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.

6. The life cycle of a Sun-sized star

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.

7. The life cycle of a massive star

It forms from a nebula and becomes a main-sequence star, fusing hydrogen into helium.
It then expands into a red supergiant.
The star explodes as a supernova.
The core is left behind as a neutron star, or as a black hole if the star was massive enough.
Elements heavier than iron are made in the supernova, which spreads them through space.

8. Time for the radio signal to reach the ground

Answer: Use $\text{time} = \dfrac{\text{distance}}{\text{speed}}$ . First change the height to metres: $540\,\text{km} = 5.4 \times 10^{5}\,\text{m}$ . Then $t = \dfrac{5.4 \times 10^{5}}{3.0 \times 10^{8}} = 1.8 \times 10^{-3}\,\text{s}$ , which is $1.8\,\text{ms}$ .

9. Time for light to reach the Earth from the Sun

Answer: $t = \dfrac{d}{v} = \dfrac{1.5 \times 10^{11}}{3.0 \times 10^{8}} = 500\,\text{s}$ , so $5.0 \times 10^{2}\,\text{s}$ to 2 significant figures. That is about 8 minutes.

10. Time for light to travel from the Moon

Answer: $t = \dfrac{d}{v} = \dfrac{3.84 \times 10^{8}}{3.0 \times 10^{8}} = 1.28\,\text{s}$ .

11. How many times further Jupiter is

Answer: $\dfrac{7.8 \times 10^{11}}{1.5 \times 10^{11}} = 5.2$ . Jupiter is 5.2 times further from the Sun than the Earth is.

12. Why the telescope does not float because of no gravity

Gravity acts between all masses, so the Earth pulls on the telescope even in space. The force is weaker far from the Earth, but it is not zero.
The telescope is falling towards the Earth all the time, and it is also moving sideways.
The sideways motion and the pull of gravity together keep it moving in a circle. It falls around the Earth instead of into it.

13. How the velocity changes while the speed stays the same

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

14. Why the radius must change with the speed

A stable circular orbit needs a speed that matches its radius.
If the speed of the object changes, it 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.