9.6 Earth’s gravity pulls objects to the centre of the Earth

9.6

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.

Video 9.6 Geomagnetism

mass the amount of matter in a substance, usually measured in kilograms; the mass of an object never changes, even in space

Figure 1 The large mass of the Earth can pull objects to its centre.

Gravity

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 fi rst 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.

weight a measure of the gravitational pull on an object

The difference between weight and mass

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 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

Figure 2 The Earth pulls base jumpers towards its centre, 6371 km below.

Earth Moon

Mass = 1 kg Weight = 9.8 N Mass = 1 kg Weight = 1.6 N

Jupiter

Mass = 1 kg Weight = 23.6 N

Figure 3 The mass of a brick does not change, but its weight is affected by gravity.

Jupiter it would weigh 23.6 newtons. On the Moon, the brick would weigh approximately 1.6 newtons because the Moon is small and has less pull force.

Gravity changes

As you move away from the Earth, the pull force slowly decreases. This means that if you stand on a chair your weight will have decreased slightly. Most scales will not be sensitive enough to measure this small change. However, if you stood on the top of Mount Everest, you would be several kilometres further away from the centre of the Earth. As a result, your weight (the amount of pull force the Earth exerts on you) is 0.25 per cent less than if you were at sea level. Gravity is not the same for every object. Objects with a larger mass experience a greater pull than objects with less mass. This means a 3 kg bowling ball feels a stronger pull force than a basketball that is lighter does. Does this mean that the bowling ball will fall faster than the basketball?

If you do this experiment, you will fi nd that both balls hit the ground at exactly the same time. This is not what most people expect to happen. Logic might suggest that heavier things fall faster. You may need to do the experiment a few times until you believe it. The heavy bowling ball needs more force to start it moving than the basketball. This offsets the larger pull, so both balls accelerate at the same rate and hit the ground at the same time.

9.6 Check your learning

Remember and understand

1 Contrast mass and weight. 2 Identify the person who fi rst described gravity. 3 If a half-full water bottle was dropped from the top of a fl ight of stairs at the same time as a full bottle of water, identify which bottle would hit the ground fi rst. 4 True or false? The pull of the Earth is stronger on an elephant than on a feather.

Mass = 120 kg Weight = 120 × 10 = 1200 N

Mass = 120 kg Weight = 200 N

Figure 4 A person who wants to lose weight could go to the Moon. A person who wants to lose mass could take their shoes off.

Figure 5 A bowling ball has more mass than a basketball and therefore takes more force to start it moving. The Earth pulls more on the bowling ball causing both balls to fall at the same speed.

Apply and analyse

5 An astronaut on the Moon dropped a feather and a hammer at the same time.

There is very little atmosphere on the

Moon to slow down objects. Explain why the feather and hammer hit the ground at the same time.

Evaluate and create

6 Building a settlement on the Moon has been suggested several times since

Neil Armstrong fi rst walked on the

Moon. Compare the advantages and disadvantages of building such a structure in a low-gravity environment.