The three see-saws here have the point of pivot at different positions. In the one with the pivot in the middle, people of equal weights manage to lift effortlessly. In the other two, the one weighing less can lift the person with greater weight up. To achieve this, the person weighing more must be seated at the end nearer to the pivot.
A see-saw is a lever pivoted at a point known as Fulcrum. The lever is free to move up and down about this point. Generally, the see-saw we encounter in the amusement parks have their fulcrum at the centre. This enables kids of comparable weights to play on it. Suppose we have two weights that differ appreciably. Then the person who weighs less cannot lift the heavier one sitting at the opposite end. In our exhibit here we make this possible. This is done by shifting the position of the fulcrum from the centre to a point little away so that one end of the see-saw is closer to the fulcrum than the other. Next, the question is about the position of the two people. Can they sit at either end and still the heavier person be lifted? No. The heavier person must sit at the end that is closer to the fulcrum. The scientific principle is as follows.
Let us first assume that just one person sits at one of the ends. Naturally, that end goes down while the opposite end goes up. The lever experiences a rotating force because of the pivot. In order to bring the lever into equilibrium condition, some weight has to be placed at the other end. How do we decide how much weight to be placed to counterbalance the weight on the other side. What is to be balanced is not the weight alone but the product of the weight and the distance of the weight from the fulcrum. If the fulcrum is at the centre, the fulcrum distance from either end is the same. That is, we need equal weights on either ends to balance. Suppose the fulcrum is closer to one of the ends. Then, in order to balance, we need to place a smaller weight at the end farther from the fulcrum and larger weight at the nearer end.
In our exhibit, we have three see-saws with fulcrum in different positions. So, people who weigh less can lift people who weigh more.
There are a number of applications of this idea, which date to third century BC. Archimedes, of Archimedes Principle fame, did, among many other things, extensive work on levers. Such was his appreciation for the utility of levers that he once remarked.
" Give me a fulcrum on which to rest a lever and i will move the earth "
Let us assume that the fulcrum is one metre from the earth. The mass of the earth is 6 X 1024Kg. Assuming that Archimedes could lift 60Kg by one metre, then we find that Archimedes would need a lever 1023 metre long.