Centre of gravity
Stand up. Stand on one leg. Shut your eyes. You should start to wobble. This is owing to your muscles pulling on your bones. As they pull they are sending messages to a part of your body which tries to keep your body balanced. This place is in your ears. When the part concerned with balance detects a wobble, it sends messages to other muscles to try and correct it. This may work for a time or it may make the body wobble even more. The reason for the wobble is that the body is trying to keep its centre of gravity acting directly through your foot. If your body moves too far one way, your centre of gravity no longer acts through your foot and you fall over.

To find out more about this balancing act it is best to think about a rectangular block of wood. You may like to find a thick book and use it as the block to help you follow the explanation. Think of the block as if stood on end on a table. Right in the middle of the block is its centre of gravity. This is the place where the whole weight of the block seems to act from. In studies on the centre of gravity the lowest part of an object is called its base. In the rectangular block the surface touching the table is its base. As the block stands there you can imagine an arrow running vertically downwards from the centre of gravity through the middle of the base.

What happens when you tip the block a little to one side? Imagine that you are tipping the block to the left. This makes the centre of gravity move towards the left. Keep thinking of the weight of the block as that arrow acting vertically downwards from the centre of gravity and you can imagine it creeping towards the bottom left corner on which the block is turning. If you only tip the block a short way the weight will still act through the base. This means that if you then let go of the block the weight will pull the rest of the block down on the base again and the block will stay upright.

What happens to make the block topple over? Imagine you are tipping the block to the left again but this time push it a long way. When you do this, the centre of gravity passes over the left-hand corner. The weight of the block now acts vertically downwards through the side of the block and not through the base. If you let the block go now, the weight will pull down on the side and not the base and the block will topple over.

Now let's think about how you balance. Stand up and put your feet about thirty centimetres apart. Close your eyes. You should find that you do not wobble. The reason for this is that your two feet wide apart provide the large base for your body. This means that there is plenty of space in which your centre of gravity can move without your weight acting through the side of your body and pulling you down. Now put your feet together and close your eyes. You should feel your body a little more. This happens because your feet now only make a small base and there is much less space in which your centre of gravity can move before your weight acts through your side and pulls you down. When you stood on one foot you had an even smaller base so the body had to work even harder to keep you from falling over.

Finally sit cross-legged on the floor and close your eyes. You should find that you could sit there comfortably for a long time. There are two reasons for this. Your seat and the sides of your legs and feet form a very large base and your centre of gravity is nearer your base. If the centre of gravity of an object is near its base, the object is more difficult to tip over. If you don't believe it, think of that block again but imagine it not stood on end but lying on its long side. When a block is arranged in this way it must be tipped much further before it falls over.

Check out your ideas about the centre of gravity now by looking at the objects around you. Divide them into those which have a low centre of gravity and a large base, and those that have a high centre of gravity and smaller base. You could predict which would be easy to topple over and which would be difficult but don't try it - it could be dangerous and expensive!

Why should you shut your eyes when you try the balancing experiments?
Your brain uses information from your eyes and from part of your ears to help you balance. If you close your eyes the brain loses that source of information and so it becomes more difficult to balance. If you don't believe it stand on one leg with your eyes open for a short time then close your eyes. You should find that you soon fall over. Shutting the eyes just makes the experiments work faster.

How does a tightrope walker balance?
By using a long pole. If you were a tightrope walker and simply stood on the rope you would have a high centre of gravity and a very small base. This means that you could easily topple over. If you hold a long pole, you and the long pole become just one object. Your centre of gravity is lowered as the pole tips down at either end and your base is made enormous due to the length of the pole. Even if you start to wobble slightly you can move the pole about to correct your centre of gravity so that it always remains above the rope and not on either side.

Why is it more difficult to knock over half empty bottles than full ones?
The centre of gravity is the point from which the weight of the bottle seems to act. Weight is the force produced by the pull of gravity on all the matter in the bottle. Liquids contain more matter than gases so when the bottle is full a large mass is spread out right to the top of the bottle and this produces a high centre of gravity. When the bottle is half empty most of the mass of the bottle is in the lower half and this makes the centre of gravity lower. Another name for the centre of gravity is the centre of mass. If an object has most of its mass round its base it will have a low centre of gravity and will be difficult to tip over.

Why do many kinds of lamp have a heavy base?
Most of the mass of the object is concentrated in the base and this gives the object a low centre of gravity so that it is very difficult to knock over.

Why are racing cars lower than ordinary cars?
They are lower so they have a lower centre of gravity. When they go into a corner at speed there are forces which try to tip the car over but because of its low centre of gravity it remains upright. If you watch rally cars you sometimes see them tip over. The reason for this is that they have higher centres of gravity than racing cars and cannot take corners at the same high speeds.

Does a double-decker bus have a high centre of gravity?
Yes, it does. It is in greater danger of toppling over than a single-decker bus. Double-decker buses are tested on a machine that tips them to see how far they can be tipped before they fall over. Sandbags are put in the bus to represent the weight of the passengers.

Why are passengers not allowed to stand on the top deck of a double-decker bus?
In busy times, if passengers were allowed to stand on both decks of a double-decker bus, it would raise the centre of gravity of the bus and make it likelier to topple over. By allowing passengers to stand only on the lower deck this concentrates the mass of the bus and passengers closer to the ground, so the centre of gravity is lower and the bus is less likely to topple over.

Are any other vehicles tested to see how they topple over?
Yes. Tractors are tested. It is important for a tractor to have a low centre of gravity because often they are used on sloping land. If a tractor has a high centre of gravity and is driven along a steep slope it could topple over.

Why are some bridges closed to high-sided vehicles in windy weather?
High-sided vehicles, such as removal vans, can have a higher centre of gravity than other vehicles. Their high sides also present a large area on which the wind can blow. If a high-sided vehicle crossed a bridge in very windy weather, the wind might push strongly on its side and its high centre of gravity may help to topple it over.

Is the position of an aeroplane's centre of gravity important?
Yes, it is. The aeroplane is designed so that its centre of gravity is over the wheels. This means that when the aeroplane takes off, it does not tip forwards or tip too far backwards and crash.