Science

Forces - Gravity

Today we will introduce gravity. This force is such a part of our lives that not to have gravity would mean that life could not exist for us.

Gravity
Gravity is a force that for us is always directed downwards. But to say that gravity acts downwards is not correct. Gravity acts down, no matter where you stand on the Earth. It is better to say that on Earth gravity pulls objects towards the centre of the Earth. So no matter where you are on Earth all objects fall to the ground
See the diagram below: What is gravity?
Gravity is a force that attracts objects together. On earth this force attracts everything to Earth.

The strength of gravity.

The Earth is a very large object and it is also very heavy. This means that it has got a strong gravitational field.
The moon is smaller than the Earth and is not as heavy.
Would you expect its gravity to be stronger or weaker than the Earth's gravity?

Weaker. The moon's gravity is only 1/6th has strong as the Earth's.
Now try this.
The Sun is a lot bigger than the Earth and is extremely heavy.
From what we have said already do you think that gravity on the Sun is stronger or weaker than that on the Earth?

A lot stronger.

Weight and Mass.
We have a problem here because of the way we use words.
We often ask how much do we weigh? And we answer in kilograms. This is not right.
Our mass is in kilograms but our weight is related to the strength of gravity.
We should really say that our weight is in Newtons.
This is really important because if we went to the moon our mass would NOT change but our weight would.

Look at the table:

 Earth Moon Mass Weight Mass Weight 60 kg 600N 60 kg 100N 80 kg 800N 80 kg 133N 100kg 1000N 100 kg 166N

From the table you can see that you mass is the same on both the earth and the moon but your weight is different. You would appear lighter on the moon.

The moon's gravity is so weak that it won't hold any gas for an atmosphere.
It's a good thing that the Earth's gravity is stronger or we would have no air to breath, it would float off into space with us!

Nasty Newtons.

Now we have found out about our weight being measured in Newtons we have to change the way we measure ourselves.
Luckily for us Newtons are very easy to deal with. Look at the next table and try and work out a way of changing from kilograms to Newtons.

 Earth Mass Weight 2 kg 20N 4 kg 40N 6 kg 60N 500g 5N 200g 2N 50 kg 500N 100 kg 1000N

Can you see the pattern?

We multiply our kg by 10.
Or we multiply our mass X acceleration due to gravity.

So 1 kg = 1 X 10 = 10 Newtons
20kg X 10 = 200 Newtons
Do you see why if you go into space away from Earth's gravity we say that you are weightless?

If I weigh 1000N on Earth, what would I weigh in space if there was no gravity?

On Earth my weight is 100kg X 10 = 1000N.
If there is no gravity my mass is still 100kg X 0 = 0.
Well, that is about it for our introduction to gravity.

Review
Remember:

• Our weight is measured in newtons;
• 1 kg = 10newtons;
• 1 Newton = 100g;
• We find our weight by multiplying our mass (in kg) by the acceleration due to gravity (10);
• Everything on earth is attracted to the ground;
• The strength of gravity is determined by the size of the Earth and its mass;
• The moon is smaller and lighter than the Earth so its gravity is weaker (1/6 of the Earth's);
• Everything is affected by gravity;

Next time we will look at air resistance and upthrust.