Gravity:

The force that keeps us from falling off

Have you ever wondered why things fall down and not go up? Have you also wondered why people on the Earth don't fall off?

If you have heard of Sir Isaac Newton, you would have probably heard the story about an apple falling almost on his head, causing him to think of this very same idea! Every object in the Universe attracts other objects with a force that is proportional to the masses of the objects and inversely (in reverse) proportional to the squared distance between objects.

Since the Earth is massive, it attracts us (no matter where we stand) with a force strong enough to keep us from falling off. We call this force gravity.

We can give a definition to gravity as follows; gravity is an attractive force between any two objects in the Universe. The force attracting any two bodies is proportional to the product of their masses and is inversely proportional to the squared distance between them.

The attractive force of gravity acts between the centres of the two objects. In the case of people standing on the Earth's surface, the effect of gravity is to attract us towards the centre of the Earth.

As a result, no matter where you stand on the Earth, you do not fall off. Gravity is also the reason why the Moon (and satellites) orbits the Earth and why we orbit the Sun.

Then, why does'nt the Moon crash onto the Earth? In fact, the Moon is constantly falling towards Earth; it is just that it keeps missing. With an initial motion along the path of the orbit, the Moon can continually fall towards Earth without ever reaching it.

In less than half a century, we have learned so much about the force of gravity, the Earth, and the Universe we live in.

We have gone from a time when the idea of space travel was just that, an idea, to the present, when man has landed on the Moon and Rovers have explored Mars.

It is interesting to think about some of the predictions that were made in the past, and to compare those predictions with reality. For example, in the 19th century, Jules Verne wrote a novel called Around the Moon.

He thought that a traveller flying in space would not experience the pull of gravity! Little did he know his prediction, which was once only fiction, will become fact! In space, everything becomes weightless!

Ever since that book was published, the image of astronauts floating effortlessly in their space ships has inspired many people.

Do the creative artists of the present really have the ability to foretell the future?

Whether or not they do, it makes science fiction all the more interesting when you think that perhaps the imaginary technology described in these novels may one day become real. From fiction to fact, humankind's control over the force of gravity has the potential to revolutionise our world.

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The standard formula for Newton's Law of Gravity is:

Gravitational force = (G m1 m2)

Where G is the gravitational constant, m1 and m2 are the masses of the two objects for which you are calculating the force, and d is the distance between the centres of gravity of the two masses.

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Experiment - Spinning ball

What you need

* A small rubber ball
* Flat tabletop surface
* Medium-sized jam bottle or a condensed milk can with a wide mouth

What to do:

Now see if you can lift the ball up from the tabletop without touching the ball or tipping over the jar. Can you do it?

Steps:

1. Place the ball on top of the table.
2. Place the jar over the ball so that the ball is inside the mouth of the jar.
3. Start spinning the jar around in a circular motion (keeping it on the table).
4. When the ball starts spinning inside the jar, lift it from the tabletop.
5. The ball too will be lifted along with the jar from the table and it will continue to spin inside the jar until it loses its speed.

This works because the ball spinning inside the jar is trying to escape, but the jar itself forces the ball to stay inside the wall of the jar. Due to the force of the spin or speed, the ball will continue to spin until it loses its speed, gravity will pull it back to Earth, and the ball will fall from the jar.

Compiled by Janani Amarasekara