© Boardworks Ltd 2004 1 of 20 © Boardworks Ltd 2005 1 of 24 Astronomy Gravity and Space.

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Astronomy

Gravity and Space


Gravity and Newton

Gravity is the most important force in the universe!

It is an attractive force that makes apples fall from trees toward Earth and makes the planets orbit the Sun.

Isaac Newton was a scientist who was interested in forces and gravity.

Newton discovered that gravity is related to the mass and distance of objects. His work was so important that the unit of force, the Newton, is named after him.


Gravity is a force of attraction that acts between all objects that have mass. The size of the force depends on the mass of the object.

Think about it:

When you jump, the gravitational force of the Earth pulls you down. Your gravitational force is also pulling the Earth towards you – you just don’t notice this effect because your mass is so much smaller than Earth’s!

Gravity and mass

All objects produce a gravitational force. This is a massive force for huge masses such as a planet.


The force of gravity between two objects depends on the mass of the objects and also the distance between them.

Spacecraft produce a very large force, called thrust, to overcome the force of gravity.

As a spacecraft gets further away from Earth, the force of gravity gets smaller.

Why do spacecraft lose their large fuel tanks and booster rockets once they have left the Earth’s surface?

force of gravity

Gravity and distance


Acceleration of gravity

Galileo concluded hundreds of years ago that all objects released together fall at the same rate regardless of mass. This is called the acceleration of gravity.

At the end of the last Apollo 15 moon walk, Commander David Scott performed a live demonstration. He held out a hammer and a feather and dropped them at the same time. Because they were in a vacuum, there was no air resistance and the feather fell at the same rate as the hammer.


Mass and weight are not the same!

What are mass and weight?

Mass is the amount of matter in an object and is measured in kilograms. Mass is not a force.

Weight is a force and is caused by the pull of gravity acting on a mass . Like other forces, weight is measured in newtons and has both magnitude and direction.

Mass will have the same value anywhere in the Universe, including space.

Weight has different values depending on where you are in the Universe.


Weight and mass on the Moon

The force of gravity is 1/6 less on the Moon than it is on Earth. This is because the Moon has a smaller mass than the Earth.

However, the astronaut still has the same body and the same mass: he just weighs less because gravity is weaker on the Moon.

Any object will weigh 1/6 less on the Moon than it does on Earth.

An astronaut can also jump 6 times further on the moon because the force of gravity is less.


Weight and mass activity


The Sun the most massive object in our Solar System.

The mass of the Sun is about 333,000 times the mass of the Earth, so the Sun exerts a massive gravitational pull.

Gravity and the Solar System

The gravitational force between the Sun and planets keeps them moving in orbits around the Sun while inertia, the force keeping the planets moving in a straight line, keeps them from being pulled towards the sun.

What would happen to the planets if there was no gravity from the Sun?

© Boardworks Ltd 2004 1 of 20 © Boardworks Ltd 2005 1 of 24 Astronomy Gravity and Space.

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