GRAVITY

« GRAVITY 1.The Earth's pull In the autumn the apples on an apple tree ripen and, if they are not picked, they one by one detach themselves fromthe branches and drop to the ground.

A force must be pulling them downwards.

When you throw a ball into the air, it travels up and over inan arc, but soon falls back to the ground.

Again, a force must be pulling it downwards.

This downwards force is gravity.

It is the attraction,or pull, the Earth exerts on everything on or near it.

All the other heavenly bodies exert gravity in a similar way.

Gravity is literally whatholds the universe together.

One of the first people to investigate the Earth's gravity was the Italian scientist Galileo, in the early 1600s.Galileo lived in Pisa at the time, and is supposed to have carried out an experiment from the top of the famous leaning tower there.

Hedropped two weights from the tower, a light one and a heavy one.

Scientists of the time believed that heavy objects fell faster than lightones.

But when Galileo dropped his weights from the tower, they both hit the ground together.

Galileo proved in this experiment that,whatever their weight, bodies fall to the Earth at the same rate when they are dropped.

Prove this for yourself by dropping a golf ball(heavy) and a ping-pong ball (light) from the same height.

If you dropped a pebble over a high cliff and were able to measure its speedas it fell, you would find that it would be travelling at a speed of about 9.8 metres per second after one second.

After another second, itwould be travelling 9.8 metres per second faster; and after another second, 9.8 metres per second faster still.

And so on.

You would findthat its speed increased by 9.8 metres per second every second it was falling.

In other words, the rate of increase in the pebble's speed -its acceleration - was 9.8 metres per second per second.

And every falling body accelerates at this rate because of the Earth's pull.

We callthis the acceleration due to gravity, or g.

We saw in the golf and ping-pong ball experiment earlier that both balls hit the ground together.They fall at the same rate because gravity accelerates them equally, even though they have a different weight.

2.Falling through air Wewould expect any objects to fall the ground together when they are dropped together.

But do they? Drop an orange and a balloon together.Do they hit the ground together? You find that they don't.

The orange hits the ground before the balloon.

So our theory that all objects fallto the ground at the same rate is upset.

Clearly another force is involved here besides gravity.

And it is slowing down the balloon.

Thisforce is the resistance, or drag, of the air.

Air resists the movement of anything travelling through it.

And the bigger the object, the greateris the resistance acting upon it.

So the balloon, which is much bigger than the orange, experiences greater air resistance and is sloweddown more as it falls.

In the same way, a hammer and a feather should fall together when they are dropped, but don't because the airresistance affects the feather more.

However, if you dropped the hammer and the feather on the Moon, they should fall together becausethere is no air and therefore no air resistance.

In fact one of the Apollo astronauts, David Scott, carried out just this experiment on theApollo 15 mission in July 1971.

He held up the geological hammer he had been working with and a feather he had brought from Earth, andlet them go.

Under the pull of the Moon's gravity, they both hit the ground together.

"How about that," cried Scott, "Mr Galileo was correct!"3.Newton and the law of gravity Galileo died in 1642.

By coincidence, this was the same year that another scientific genius was born, inEngland.

He was Isaac Newton, whose theoretical and practical work transformed the natural sciences and mathematics.

He is particularlyremembered for his discovery of the laws of gravity, in a story that may or may not be true.

The story goes that one day he was sittingunder an apple tree, when an apple fell to the ground near his feet.

This set him wondering whether the force that pulled the apple to theground - that is, gravity - was the same force that keeps the Moon circling endlessly around the Earth.

He decided that it was.

The Moon istravelling through space.

If no forces acted upon it, it would travel in a straight line.

But in fact it circles around the Earth.

So there must bea force connected with the Earth that attracts the Moon and makes it travel in a circle, in orbit around the Earth.

This force must be theEarth's gravity.

You can see how gravity acts on the Moon by whirling a stone on a piece of string around your head.

Make sure you are outin the open and there is nobody about! The stone keeps travelling in a circle because you are pulling on the string.

If you let go of thestring, the stone will shoot off in a straight line.

And so it is with the Moon.

Gravity (the inward pull) keeps the Moon (the stone) travellingin a circle.

If gravity were suddenly to cease, the Moon would fly off into space in a straight line.

Newton realised that it was not only theEarth that had gravity, but every body in the universe.

The gravity of the Sun holds the planets in their orbits in the solar system.

Gravitybinds stars into great star islands, or galaxies; and galaxies into clusters and superclusters.

Gravity holds the whole universe together.Newton summed up his ideas on gravity in his universal law of gravitation: Every bit of matter in the universe attracts every other bit ofmatter with a force that depends on their masses, and inversely on the square of the distance between them.

Expressing thismathematically: the force of gravity (F) between two bodies of mass m and m and distance d apart is proportional to the product of theirtwo masses (m m ) and 1 over d squared.

We can write this as: see diagram This shows that if you double one of the masses, you doublethe gravitational force.

But if you double the distance between them, you reduce the force one quarter (one over two squared).

4.Mass andweight On the Earth, gravitational force - the force of gravity - exists between every object on the Earth's surface and the Earth itself.

It actsto pull the object downwards to the surface.

It is the force we call weight.

From Newton's law of gravitation, we see that this force isproportional to an object's mass.

The greater the mass of an object, the more the Earth attracts it, and the greater is its weight.

The terms'mass' and 'weight' are often confused.

But as you can see, they are different.

'Mass' is the amount of matter in an object.

It neverchanges.

'Weight' is the force acting on an object because of gravity.

It changes when the strength of gravity changes.

The force of gravitythat attracts an object to the Earth depends, of course, not only on the object's mass but on the mass of the Earth which is attracting it.

Sowe can say generally that for a given object, the force of gravity it experiences depends on the mass of the attracting body.

Or in otherwords, the weight of an object depends on the mass of the attracting body.

For example, the Moon is much smaller and has much lessmass than the Earth.

So its gravity is much weaker - only about one-sixth that of the Earth.

This means that objects on the Moon weighonly one-sixth what they do on Earth.

On the other hand, the planet Jupiter is much bigger and more massive than the Earth.

So its gravityis much greater - over two and a half times greater.

This means that objects on Jupiter would weigh over two and a half times more thanthey do on Earth.

So we see that a particular object would have a different weight on the Earth, the Moon and Jupiter.

But it would still havethe same mass.

Its mass is fixed.

Its weight varies according to the strength of gravity.

5.Gravity and satellites Coming back to Earth, andlooking again at Newton's law, we see that the force of gravity on an object also depends on distance (d in the formula).

This is thedistance between the object and the Earth's centre (its centre of mass).

So as you climb above the surface, gravity gets less.

But thechange is very gradual.

Only when you soar hundreds of kilometres into space does gravity weaken markedly.

And the higher you go, theweaker it becomes.

This explains why satellites orbiting higher up do not have to travel so fast to stay in orbit.

In their orbiting spacecraft,we talk of astronauts being in a state of zero-g (meaning no gravity).

But of course this is not true.

Gravity still acts in orbit.

If it didn't, thespacecraft would fly off into space.

We also call the 'zero-g' condition 'weightlessness', because nothing in orbit appears to have anyweight.

Weight is the downwards pull acting on an object because of gravity.

And because gravity is still present in orbit, objects are stillbeing pulled downwards.

They are falling towards the Earth.

But you can't measure their weight - the downwards pull on them - with a pairof scales, because the scales will be falling too!. »