Lesson 3 of 6
Gravity and orbits
Goal: Describe how gravity depends on mass and distance and keeps objects in orbit.
Gravity pulls every object towards every other object. This pull holds moons, planets and satellites in orbit.
- More mass, more pull. The Earth has far more mass than the Moon, so its gravity is stronger.
- More distance, less pull. The further an object is from the centre of the Earth, the weaker the Earth’s pull on it.
- Gravity holds things in orbit. The Sun’s gravity keeps the planets and the comets moving round it. The Earth’s gravity does the same for the Moon and for satellites.
Natural satelliteA moon. The Moon is the Earth’s natural satellite.
Artificial satelliteA machine that people have put into orbit. Satellites watch the weather, take pictures of the Earth and carry phone and TV signals.
Your turn
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Key words in this lesson
Mass The amount of matter in an object, measured in kilograms
Gravity A force that pulls any two objects towards each other
Orbit The path of a planet, moon or satellite around a larger body
Natural satellite A moon: a body, not made by people, that orbits a planet
Artificial satellite A machine that people have put into orbit
Planet A large ball of rock or gas that orbits a star