11.0 Introduction
Figure 11.1
The illustration shows a swing bridge over a canal. It can be raised to allow barges and boats to pass. It is operated by hand, even though it is very heavy.
How is this possible?
The bridge depends on the turning effects or moments of forces. To understand these, you might find it helpful to look at a simpler situation.
Two children sit on a simple see-saw, made of a plank balanced on a fulcrum as
in figure 11.2. Will the see-saw balance?
Figure 11.2
If both children have the same mass and sit the same distance from the fulcrum, then you expect the see-saw to balance.
Now consider possible changes to this situation:
(i) If one child is heavier than the other then you expect the heavier one to go down;
(ii) If one child moves nearer the center you expect that child to go up.
You can see that both the weights of the children and their distances from the fulcrum are important.
What about this case? One child has mass 35 kg and sits 1.6 m from the fulcrum and the other has mass 40 kg and sits on the opposite side 1.4 m from the fulcrum (see Figure 11.3).
Figure 11.3
Taking the products of their weights and their distances from the fulcrum, gives
A: 40g × 1.4 = 56g
B: 35g × 1.6 = 56g
So, you might expect the see-saw to balance, and this indeed is what would happen.