Gravity is a pulling force

Every object with mass pulls on every other object. This pull is the force of gravity. The bigger the mass, the stronger the pull — and the closer the objects, the stronger the pull.

• The Earth is huge, so it pulls everything towards its centre — that's why things fall down.
• The Sun is far more massive still, so its gravity holds the whole solar system in orbit.
• Gravity acts between the Earth and the Moon, and between the Earth and the Sun — it reaches across empty space.

Key idea: gravity gives objects their weight. Weight is not the same thing as mass — that's the next screen.

Mass and weight are different

Mass is the amount of "stuff" in an object. It is measured in kilograms (kg) and it never changes — you have the same mass on Earth, on the Moon or floating in space.

Weight is the force of gravity pulling on that mass. It is measured in newtons (N) and it changes depending on how strong gravity is where you are.

Watch out: a common mistake is to say your mass changes on the Moon. It doesn't — only your weight does, because the Moon's gravity is weaker.

Mass or weight?

Weight = mass × g

To work out the weight of an object we multiply its mass by the gravitational field strength, g:

On Earth, g is about 10 N/kg. That means every 1 kg of mass is pulled down with a force of about 10 N.

Tip: read the question carefully — kg is a clue you have mass, and N is a clue you have weight (a force).

Your turn — weight on Earth

g is different on other worlds

The gravitational field strength g depends on the mass of the planet or moon you are standing on. More massive worlds have a bigger g, so the same object weighs more there.

Your turn — weight on the Moon

Your turn — find the mass

The Sun is a star

The Sun is not special because it is a different kind of object — it is a star, just like the twinkling points of light you see at night. It only looks so big and bright because it is very close to us compared with all the other stars.

Watch out: the Sun is a star, not a planet. Planets (like Earth) do not make their own light — they only reflect light from a star.

Stars, galaxies and the Universe

Space is organised on bigger and bigger scales:

• A star is a giant ball of hot gas that gives out its own light — our Sun is one.
• Billions of stars grouped together form a galaxy. Our galaxy is the Milky Way.
• There are billions of other galaxies. All of them together make up the Universe.

Star, planet or galaxy?

Tap the correct type for each object.

Day and night

The Earth spins on its axis once every 24 hours. The half of the Earth facing the Sun has day; the half facing away has night.

Watch out: the Sun looks like it "rises in the east and sets in the west", but the Sun is not really moving across the sky — it is the Earth's spin that makes it look that way.

Why do we have night?

The seasons and the Earth's tilt

The Earth's axis is tilted by about 23.5°. As the Earth orbits the Sun over a year, this tilt means different parts of the Earth get sunlight more directly at different times.

In summer the Sun is higher in the sky, days are longer, and sunlight hits the ground more directly, so it is warmer. In winter the Sun is lower, days are shorter, and light is spread out more thinly.

Seasons are NOT about distance

A very common misunderstanding is that summer happens because the Earth is closer to the Sun. This is wrong.

If distance caused the seasons, the whole planet would have summer at the same time. But when it is summer in the UK it is winter in Australia — the two hemispheres have opposite seasons at once.

What causes the seasons?

The Sun and stars appear to move

Because the Earth is turning, the Sun and stars appear to sweep across the sky:

• Each day: the Sun appears to rise in the east, arc across the sky, and set in the west. The stars do the same at night.
• Across the year: as the Earth orbits the Sun, we see different stars and constellations in the night sky in different seasons.

The light year

Space is enormous. Distances between stars are far too big to measure sensibly in kilometres, so astronomers use the light year.

A light year is a unit of distance, not time. Because light is so fast, one light year is about 9.5 million million km — a huge distance.

• Light from the Sun takes about 8 minutes to reach us.
• The next nearest star is over 4 light years away.
• When you look at a distant star, you see light that left it years ago — you are looking into the past.

Watch out: a light year measures how far, not how long. "The star is 4 light years away" means a distance, even though the word "year" is in it.

What is a light year?

The order of the planets

Our solar system is the Sun plus everything held in orbit by its gravity — the eight planets, their moons, and lots of smaller rocks and ice. The planets, in order out from the Sun, are:

Remember: the planets do not make their own light — they only shine because they reflect sunlight. Only the Sun (a star) makes its own light in our solar system.

Order the planets

Tap the planets in order, starting nearest the Sun.

Putting it together

The key ideas to know

You've covered the whole KS3 Space Physics topic. Press Finish to see your score.