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Space, gravity and the seasons
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Gravity holds the Moon in orbit round the Earth and the Earth in orbit round the Sun. It also gives you your weight.
Mass and weight
Mass is how much matter something has, in kilograms (kg). It is the same everywhere.
Weight is the force of gravity on that mass, in newtons (N).
weight = mass × gravitational field strength (W = m × g)
On Earth, g is about 10 N/kg. It is different on other planets and on stars. On the Moon it is about 1.6 N/kg, so you would weigh less there.
Weight is the force of gravity on that mass, in newtons (N).
weight = mass × gravitational field strength (W = m × g)
On Earth, g is about 10 N/kg. It is different on other planets and on stars. On the Moon it is about 1.6 N/kg, so you would weigh less there.
Our place in the universe
Gravity attracts every mass to every other mass. It keeps the Moon orbiting the Earth and the Earth orbiting the Sun.
The Sun is a star. It is one of billions of stars in our galaxy, the Milky Way, and there are billions of galaxies in the universe.
Distances in space are huge, so we use the light-year: the distance light travels in one year.
The Sun is a star. It is one of billions of stars in our galaxy, the Milky Way, and there are billions of galaxies in the universe.
Distances in space are huge, so we use the light-year: the distance light travels in one year.
Days and seasons
The Earth spins on its axis once a day, giving day and night.
The axis is tilted. When your half of the Earth (hemisphere) is tilted towards the Sun, it is summer: days are longer and the Sun is higher in the sky, so its rays are more concentrated.
When your hemisphere tilts away, it is winter, with shorter days. When it is summer in the UK, it is winter in Australia.
The axis is tilted. When your half of the Earth (hemisphere) is tilted towards the Sun, it is summer: days are longer and the Sun is higher in the sky, so its rays are more concentrated.
When your hemisphere tilts away, it is winter, with shorter days. When it is summer in the UK, it is winter in Australia.
An astronaut has a mass of 70 kg. Find her weight on Earth (g = 10 N/kg).
- W = m × g
- W = 70 × 10
Answer: 700 N
Weight (N) = mass (kg) × g. On Earth g = 10 N/kg. Mass stays the same; weight changes with g. The Sun is a star in the Milky Way. A light-year is a distance. The tilt of the Earth's axis causes the seasons.
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