🌍 Its mass
The more mass a planet or moon has, the stronger its pull. A planet with a lot of mass has a large g. A small, light moon has a small g.
Motion in the Universe » The Universe and Gravity
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 8.1, 8.2, 8.3
- Put a planet, a solar system, a galaxy and the universe in order of size
- Know that our solar system is in the Milky Way galaxy
- Explain why gravitational field strength, g, is different on different planets and moons
- Work out weight on other worlds, remembering that mass never changes
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Unlock This CourseThe universe, galaxies and the solar system
Astrophysics starts with where we are. The scales are huge, so it helps to build them up one step at a time.
Our planet, Earth, is one of the planets that orbit the Sun. The Sun and everything that orbits it make up our solar system. The Sun is a star, and it is only one of billions of stars. Stars are grouped together in enormous collections called galaxies.
Key terms:
- Galaxy: a large collection of billions of stars.
- Milky Way: the galaxy that contains our solar system.
- Universe: a large collection of billions of galaxies.
Putting them in order of size
Each one fits inside the next, like a set of boxes. From smallest to largest:
| Order | Object | What it is |
|---|---|---|
| 1 (smallest) | Planet | A world that orbits a star, such as Earth |
| 2 | Solar system | The Sun with all the planets and other bodies that orbit it |
| 3 | Galaxy | Billions of stars, such as the Milky Way |
| 4 (largest) | Universe | Billions of galaxies |
Common mistakes
- Mixing up the solar system and the galaxy. The solar system is just one star (the Sun) and its family. The galaxy has billions of stars.
- Thinking the Sun is special. Our Sun is simply one of the billions of stars in the Milky Way.
- Saying the Milky Way is the universe. It is only one of the billions of galaxies.
The units used in this topic
Here are the units you will meet in astrophysics. Most you have already used.
| Quantity | Unit | Symbol |
|---|---|---|
| Mass | kilogram | kg |
| Distance, radius | metre | m |
| Speed | metre/second | m/s |
| Acceleration | metre/second squared | m/s2 |
| Force, weight | newton | N |
| Time | second | s |
| Gravitational field strength | newton/kilogram | N/kg |
Why gravitational field strength is different on other worlds
Every planet and moon pulls objects towards its centre. The strength of that pull on each kilogram of mass is the gravitational field strength, g. On Earth, g is 10 N/kg (the value we use in this course). On other worlds it is different. Two things decide how big g is:
🌙 Its radius
The radius is the distance from the centre to the surface. The closer you are to the centre of the planet, the stronger the pull. So for the same mass, a smaller radius means a larger g.
The Moon has much less mass than the Earth, which makes its pull weaker. Its radius is smaller too, which pushes g up a little, but the small mass wins overall. That is why g on the Moon is only about 1.6 N/kg.
| World | g (N/kg), approximately |
|---|---|
| Moon | 1.6 |
| Mars | 3.7 |
| Earth | 10 |
| Jupiter | 25 |
Jupiter has a huge mass, so its g is much larger than Earth's, even though it is much wider.
Mass stays the same, weight changes
Remember from Weight, Mass and Gravity that weight is a force, found with W = m × g. Mass is the amount of matter in an object. If you take an object to another world, you have not added or removed any matter, so its mass does not change. But g changes, so its weight does change.
Worked example 1
A student has a mass of 60 kg. Find the student's weight on the Moon (g = 1.6 N/kg) and on Jupiter (g = 25 N/kg).
Moon: W = m × g = 60 × 1.6 = 96 N
Jupiter: W = m × g = 60 × 25 = 1500 N
The mass is 60 kg in both places. Only the weight changes.
Worked example 2
A rock sample weighs 8.0 N on the Moon (g = 1.6 N/kg). Find its mass and its weight on Earth.
Rearrange: m = W ÷ g = 8.0 ÷ 1.6 = 5.0 kg
On Earth: W = m × g = 5.0 × 10 = 50 N
Common mistakes
- Saying that mass changes on the Moon. Only weight changes.
- Saying that a planet's pull depends only on how big it looks. Mass and radius both matter.
- Leaving out the unit. Weight is in N, mass is in kg, g is in N/kg.
Exam-style question
A space probe has a mass of 150 kg. The gravitational field strength on Earth is 10 N/kg and on the Moon it is 1.6 N/kg.
(a) Calculate the weight of the probe on Earth. [2 marks]
(b) Calculate the weight of the probe on the Moon. [2 marks]
(c) State the mass of the probe on the Moon. [1 mark]
(d) Explain why g on the Moon is less than g on Earth. [2 marks]
Model answer
(a) W = m × g (1) = 150 × 10 = 1500 N (1)
(b) W = 150 × 1.6 (1) = 240 N (1)
(c) 150 kg (1)
(d) The Moon has much less mass than the Earth (1), so its gravitational pull on each kilogram is weaker (1).
Exam tip
In a "state the mass" question on another planet, the answer is the same number as on Earth. Do not multiply by g.