Gravity
Hold up a pencil then let it go. What happens? As soon as you release your grip the pencil speeds away towards the floor and hits it. The pencil was propelled by gravity. When you let go of the pencil, gravity did not just give a quick tug on the pencil: it pulled on the pencil all the way down to the floor. If a force keeps working on an object, the object keeps speeding up or accelerating. The pencil only fell for a short distance, so its acceleration is almost impossible to see but, if you tried the experiment with a jug of water, you could see the effects of acceleration much better.

Take a jug of water and hold it just above a bowl. Pour the water into the bowl and see the steady column of water flow from the jug spout. Listen to the single sound of the column of water hitting the surface of the water as the bowl fills up. Now repeat the experiment but this time hold the jug high above the bowl. If the water just received a quick tug from gravity and was left to fall, it should stay in a column and make a single sound as it enters the bowl. When you look at the water flowing from the jug you see something different. The water starts as a column but then starts to split up and falls into the bowl as drops making many separate sounds. The reason for this change is the continuous pull of gravity on the water.

All substances are made from particles of matter. Water is made from particles which have a force which holds them close yet allows them to slide over each other. When the water falls from a height, the acceleration causes the forces between some of the particles to fail to hold the particles together and the water splits up into drops. This is best seen at a waterfall where the water can fall a great distance.

From watching objects fall to the ground you would think that the pull of gravity just works one way - from the object to the Earth. In fact there is a pull of gravity between any two objects in the universe. The pull of gravity that an object has depends on its size. A small object has a small pull of gravity and a large object has a large pull of gravity. The pull of gravity between two small objects such as a beetle and a pebble is so weak that it makes no effect on them. The gravity between two large objects such as stars is so strong that it can pull both towards each other and make them spin round each other. When you consider the gravity between a large object and a small one something different happens. The large object exerts a strong force of gravity on the small object and the small object exerts a small force of gravity on the large one. As the large object has the larger force of gravity it pulls the smaller object to it. This is what happens between the Earth and everything that is on its surface or in the air and a little way out into space.

The Earth's gravity acts to pull everything down to the centre of the planet. Some people think that it just pulls objects down to the surface but if you dig a hole and let something fall above it, the object drops in showing that the force of gravity works below the Earth's surface. As you move further away from an object its force of gravity becomes weaker. For example, the force of the Earth's gravity pulling on a spacecraft gets smaller as the spacecraft moves away from the Earth. The force of gravity on a mountaintop is slightly smaller than the force of gravity at sea level simply because the mountaintop is further away from the centre of the Earth!

It's time to pick that pencil up. As you bend down some of your muscles relax so that gravity can take over and pull your arm and hand down towards the floor. When you have hold of the pencil, other muscles in your body pull on your bones and exert a force against gravity, which makes you sit up again. All your life your muscles work against the force of gravity to help you move. You will never know what it is like to be without the Earth's pull of gravity unless you get the chance to travel into space.

What are the smallest objects that gravity pulls towards the Earth?
These are probably the particles in the air. Air is made from a mixture of gases. Each gas is made from particles, which can move about freely. The main gases in the air are nitrogen, oxygen, carbon dioxide and water vapour. Another name for the mixture of gases around a planet is the atmosphere.

Do other planets have atmospheres?
Yes, they do. Some planets such as Venus have a very thick atmosphere full of clouds. Others such as Mars have a thin atmosphere. The Moon, an object that is considered by some scientists to be a double planet with the Earth, has a force of gravity six times weaker than the Earth's gravity and does not have an atmosphere. This means that astronaut footprints left in the dust on the surface years ago are still there. This is due to the absence of an atmosphere in which winds could blow and move the dust away.

Would your weight change if you went to the Moon?
Yes, it would. As the Moon would pull on your body with a force of gravity six times less than on the Earth, you would weigh six times less than you do on Earth. On Mars the pull of gravity is three times less than Earth so you would weigh three times less. If you could visit Jupiter you would find that you weighed over twice as much because the pull of Jupiter's gravity is over twice as strong as the pull of Earth's gravity.

Would your mass change if you visited the moon or another planet?
No. Your mass is a measure of the amount of matter in your body. This would stay the same. You would not get six times smaller on the Moon or over twice as large on Jupiter if you visited those places. Weight is not the same as mass. Your weight is the force produced when a force of gravity (of a planet or a moon) pulls on the mass of your body.

Does the Sun have a force of gravity?
Yes, it does. This force is huge. It is so strong that it holds the planets in their orbits in the Solar system.

Why don't the planets fall into the Sun?
The Solar system formed from a cloud of gas. It is believed a star exploded close to the gas cloud and started the particles moving. You may have expected the particles to fly away in all directions but the large number of particles at the centre of the gas cloud exerted a force on the other particles and made them turn rather than go off in a straight line. As the particles turned they joined together. At the centre of the cloud the Sun formed and further out the planets formed. Today, the force that set the particles moving causes the planets to move forwards but this force is balanced by the Sun's gravity so the planets neither go towards the Sun nor go out into space.

Does the force of gravity of the Moon affect anything on Earth?
Yes, it does. It affects the water in the seas and oceans. The pull of the Moon's gravity makes the water rise up or bulge directly below it and also on the opposite side of the planet. These bulges move across the oceans as the Earth rotates and cause the water on the shore to rise and fall. These movements of the water on the shore are called tides. Twice a month the Sun lines up with the Moon as the Earth and Moon move round the Sun. When this happens the Moon's gravity is given a little help by the Sun's gravity and makes the tides higher and lower on the shore. These tides are called spring tides although they occur about twice a month. The smallest tides, which also occur about twice a month, are called neap tides.