1. GRAVITY AND SPACE In This Topic We Will Learn About:
Gravity and gravitational strength.
Natural and artificial satellites.
The history of the solar system.
Copernicus and Kepler.
Discoveries.
Telescopes and radio telescopes.

2. Gravity is a force that always attracts objects.
GRAVITY AND SPACE
Gravity is a force that always attracts objects.
All objects produce a gravitational force but only very large objects make a force big enough to affect us.
The greater the mass of the object, the greater the strength of its gravity.
The further apart two objects are, the lower the force of gravity between them.

3. GRAVITY AND SPACE
The Earth’s gravity attracts every object on or near the Earth.
It tries to pull them towards the centre of the Earth.
The Earth pulls every kilogram with a force of 10 Newtons.
Another way of saying this is to say the Earth’s gravity has a strength of 10 N/kg or that 1 kg weighs 10 N.
How much would a girl with a mass of 40kg weigh on Earth?
40 x 10 = 400 N

4. GRAVITY AND SPACE
Different planets, stars and moons have different gravitational strengths.
The moon has a gravitational strength of about 1.5 N/kg.
This means that every kilogram weighs only 1.5 N on the Moon.
How much would a girl with a mass of 40 kg weigh on the Moon?
40 x 1.5 = 60 N
Jupiter has a gravitational strength of about 20 N/kg.
How much would the same girl weigh on Jupiter?
40 x 20 = 800 N

5. Gravity is responsible for keeping our solar system together.
GRAVITY AND SPACE
Gravity is responsible for keeping our solar system together.
All the planets are attracted by the Sun’s gravitational pull.
This keeps them moving around the Sun in orbits and stops them moving off into outer space.

6. Many planets have moons orbiting them.
GRAVITY AND SPACE
Many planets have moons orbiting them.
Our Moon orbits the Earth because it is attracted by the Earth’s gravitational pull.
Larger planets usually have more moons than smaller planets.
Why is this?
Because the pull of gravity of a large planet is stronger than the pull of gravity of a small planet. It is the planet’s gravity that keeps its moons in orbit.
Jupiter has
more than
20 moons

7. GRAVITY AND SPACE A satellite is an object that orbits a planet.
Moons are natural satellites.
We have been putting artificial satellites into space for over 40 years.
These satellites orbit the Earth and carry out useful tasks for us.
The Earth’s gravity keeps satellites in orbit.
Some satellites are used for weather forecasting.
What else are satellites used for?
Making maps, spying on other countries, scientific research, finding out about space, communications (television and telephones).

8. GRAVITY AND SPACE
We have been finding out about the stars and planets for many thousands of years.
During this time, our ideas have changed as new technology has helped us find out new things.
About 2000 years ago astronomers believed that the Earth stayed still and that the Sun, Moon and planets travelled around the Earth.
They believed their orbits were circular.
This idea was believed until about 450 years ago.

9. GRAVITY AND SPACE Kepler
In 1543 an astronomer called Copernicus put forward the idea that the Earth and other planets orbited the Sun.
Investigations from other scientists gave evidence to support his theory.
It still took a long time for his ideas to be believed.
In 1609 an astronomer called Kepler put forward the theory that planets moved around the Sun in ellipses rather than circles.
Kepler
The model of the solar system with the Sun at the centre and planets orbiting the Sun in elliptical orbits is the model we use today.
This model explains the movements of the planets and allows us to make accurate predictions.

10. GRAVITY AND SPACE Telescope Observatory Radio telescope
Name some inventions which have helped us find out more about space.
Some examples include:
Telescopes.
Radio telescopes.
Rockets that have allowed us to investigate planets and the Moon.
Satellites carrying equipment that lets us look further into space.
Telescope
Observatory
Radio telescope
Telescope built into a satellite

11. GRAVITY AND SPACE
What does the strength of the gravitational pull between two objects depend on?
Their mass.
How far apart they are.
What does an object’s weight depend on?
Its mass.
The strength of gravity.

12. GRAVITY AND SPACE
Describe how the effect of gravity changes as a spaceship travels from the Earth to the Moon.
At first Earth’s gravity is trying to stop the spaceship from getting away from the Earth. The Moon is too far away for its gravity to have much effect.
As the spaceship travels further away from the Earth the effect of Earth’s gravity gets less.
As the spaceship gets nearer to the Moon the Moon’s gravity has more effect.
At a certain point the force of the Earth’s gravity pulling the spaceship back towards the Earth will be equal to the force of the Moon’s gravity pulling the spaceship towards the Moon.
When the spaceship lands on the Moon, the astronauts will only feel the effect of the Moon’s gravity pulling them towards the centre of the Moon.

13. Check your learning Which of these statements about gravity are true?
On the Moon a feather and
a hammer would fall at the same
rate because there is no air
resistance on the Moon.
On Earth a feather will fall more
slowly than a hammer because
gravity has less effect on it.
On Earth a feather will fall
more slowly than a hammer
because air resistance has
a greater effect on the feather.
A feather and a hammer
would float on the Moon
because there is no gravity
on the Moon.
On the Moon a feather will fall
more slowly than a hammer because gravity has less effect on it.

14. Check your learning - answers
The true statements are in purple.
On the Moon a feather and
a hammer would fall at the same
rate because there is no air
resistance on the Moon.
On Earth a feather will fall more
slowly than a hammer because
gravity has less effect on it.
On Earth a feather will fall
more slowly than a hammer
because air resistance has
a greater effect on the feather.
A feather and a hammer
would float on the Moon
because there is no gravity
on the Moon.
On the Moon a feather will fall
more slowly than a hammer because gravity has less effect on it.