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Earth’s gravity pulls objects to the centre of the Earth In this topic, you will learn that:
• Earth’s gravity can cause a non-contact force • large objects (such as planets) pull objects towards their centre.
Gravity
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mass the amount of matter in a substance, usually measured in kilograms; the mass of an object never changes, even in space
One day in 1665, a young student named Isaac Newton was sitting under an apple tree when an apple fell to the ground. ‘Why did it fall?’ he wondered. There was nothing he could see that could push it or pull it. He realised that there must be a force that pulled the apple towards the Earth. This is how Newton claimed he first had the idea of gravity. Gravity is the effect of a large object (such as a planet) warping space and time. This results in the large object (Earth) attracting everything nearby to its centre. This means people, animals and apples are pulled to the centre of the Earth. Consider Figure 1. If everyone in the picture dropped an object, those objects would fall towards the centre of the Earth. Every object that is made of matter (small particles called atoms) is able to pull other things towards it. The Earth is made up of enormous amounts of matter, allowing it to exert a relatively strong non-contact force on objects around it. Even you have weak gravity surrounding you. The Earth has much more matter than you do, and therefore the Earth’s pull force is much stronger than yours. The more matter an object has, the stronger its pull force.
would be less than that on Earth. This does not mean you are smaller. It just means the Moon is pulling you down less. Because weight is a measure of pull force, it is measured in newtons. If weight is a measure of the pull force, then how do scientists describe the amount of matter of an object? Mass (measured in kilograms) is the term used to describe how many particles or atoms make up an object. The mass of an object does not change, no matter where in the universe it is. If a brick has a mass of 1 kg on Earth, it has this mass everywhere. However, the weight of the brick will change. On Earth, the brick may weigh 9.8 newtons, but on a large planet such as
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Video 9.6 Geomagnetism
Figure 1 The large mass of the Earth can pull objects to its centre.
The difference between weight and mass
weight a measure of the gravitational pull on an object
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The Moon is made of less matter than the Earth. This means that the Moon’s ability to pull objects is much less than the Earth’s. An astronaut jumping on the Moon will be able to jump much higher than on Earth. This is because the Moon does not have as strong a pull force as the Earth. Weight is a measure of the pull force on an object. Your weight on the Moon
Figure 2 The Earth pulls base jumpers towards its centre, 6371 km below.
OXFORD UNIVERSITY PRESS OXFORD SCIENCE 7: VICTORIAN CURRICULUM No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means.
