What comes up must come down 3

Newton was also very good at mathematics and he wrote an equation to describe this mysterious force as

where

is known as ``BIG G'' or the gravitational constant. This is Newton's Law of Gravitation. Scientists have been able to measure this number to be

or rather, 6.67 hundred thousand millionths; pretty small!

Let's put some numbers into this equation.

The mass of the Earth in kilograms is 6 with 24 zeroes after it. However, your average apple is only about say 0.1 kg. When you hold such an apple, we can calculate from Newton's Law that the force of attraction between the apple and the Earth (the force of gravity) is about 1.0 Newton.

Be careful of one possible problem here. In Newton's formula given at the top of the page we need to know "the distance between objects". For gravity calculations this means "the distance between the centres of mass of the objects". For nearly spherical objects like the Earth and an apple we can assume the centre of mass is at the centre of the object. The distance between the Earth and apple is not therefore one metre (or however high you are holding the apple above the Earth's surface) but the distance of the apple's centre from the Earth's centre - about 6.38 million metres!

Now, let us say that your mass is 60 kg (a wild guess!!). Your computer in front of you is probably about 30 kg. Your face is probably about half a metre (0.5m) from the computer screen. The force of attraction in this case between you and the computer is 4.8 ten millionths (0.00000048) Newtons.

and that is over 2 million times less than the force of the Earth's gravity on the apple you are holding. No wonder you don't feel a great attraction for your computer!

But gravity does more than simply help apples drop out of the sky.