Levers
When was the last time you played on a see-saw? It was probably some time ago but you can often see young children on a see-saw in a playground in a park. A see-saw looks a simple device but how does it work?

There are two basic parts to a see-saw. There is a long piece of metal called a bar and underneath the middle of the bar is an edge on which it balances. This edge is called a pivot. The parts of the bar on each side of the pivot are called arms. What happens when someone sits on the end of each arm? If the two people are the same weight the bar balances. It stays level. This happens because the force pushing down on one end of the arm is the same as the force pushing down on the other. The people make the see-saw work by pushing on the ground in turn. When one person pushes on the ground they produce an upward force which is greater than their weight. The force pushing down on the end of the arm is less than the force pushing down on the other end of the arm so the other person sinks to the ground. When this happens, the person at the other end stops moving and their full weight is again pushing down on the end of the arm. The person now on the lower arm pushes on the ground and the force they exert is stronger than their weight. The force pushing down here is now less than the force pushing down at the other end of the arm which is in the air so the see-saw tips back again.

When one person sits nearer the pivot, something different happens. They rise into the air even though they do not push on the ground. They have not lost weight so what has happened? The answer is to do with where the people are sitting and how the arms really move. So far we have thought about the arms going up and down but really they are turning about the pivot. If you look at a see-saw from the side you can see the arm on the right turns clockwise as it sinks and the arm on the left turns anticlockwise as it sinks. The size of the turning force is not due just to the force but also due to the distance of the person from the pivot. If the person is sitting a long way from the pivot the force they can produce is large but if they are sitting close to the pivot the force is small. When one person sits nearer the pivot their force is smaller than that of the other person. This results in the other person sinking because they are putting the larger force on the bar and the person near the pivot rising because they have the smaller effect.

You can make a see-saw from a ruler and a piece of chalk. Put the chalk mid-way under the ruler to make the pivot. You can now convert the see-saw into a simple machine called a lever by moving the pivot nearer one end. This gives you a short arm and a long arm. If you put a heavy object on the short arm you may be surprised to find that you can lift it by pushing down on the end of the long arm with a small force. The reason for this is that the weight is a short distance from the pivot and only a small force needs to be applied to the end of the long arm to lift the object.

The longer you make this arm the greater its power to move heavy objects. Next time you see someone trying to open a paint tin lid by levering it up with a screwdriver, think about how this simple device works. The small force they push down overcomes the large force of friction, which is holding the lid in place.

Is a lever a machine?
Yes, it is. A machine is a device which helps you to do work. A lever helps you to use a small force to overcome a large force. It saves you energy exerting a large force.

Are there levers in a pair of pliers?
Yes, there are two levers. They are one above the other and work in different directions. The part used to grip is made from the short arms of the levers and the handles of the pliers are made from the long arms. When you grip a pair of pliers you push down on the top handle with a small force and a large force pushes upward on the lower gripping part. At the same time you push upwards on the lower handle with a small force and this makes a large force push down on the upper gripping part. The small forces that you use to grip the handle are multiplied into large forces which help the pliers to grip.

Is a pair of scissors a pair of levers?
Yes, it is. The small forces you apply to the handles are multiplied into larger forces, which move a sharp edge so it can cut through material. You may have used scissors to cut thin wire. When you do this you place the wire between the blades as close to the pivot as possible. This means that the large force on the scissors handles easily overcomes the wire and it can be cut. But if you try to cut the wire when it is close to the ends of the pliers, it will not cut because you cannot get enough force: the handles and the pliers ends are too closely balanced.

How many different kinds of levers are there?
There are three basic kinds. The lever made from a see-saw with a long arm and a short arm is one kind. In the second kind, such as a wheelbarrow, the pivot is at one end of the bar. In the third kind, the pivot is also at the end of the bar but this time the load is at the weight at the other end of the bar and the effort is between them. Your lower arm is an example of this. Raise one of your hands until your lower arm makes a right angle with your upper arm. Feel your elbow - this is the pivot. Feel your biceps muscle - this is the part of you that makes the effort. Run your finger down from your biceps muscle to the place where it is attached to the inside of your elbow. This is the place where the effort acts upwards. The weight, which acts down in your arm lever, is anything you carry in your hand.