Pulleys are a simple form of machine. They make jobs easier
to do. Pulleys are usually used to lift things.
If we ignore friction the more pulleys in a
pulley system the less effort you have to use to lift up a certain
load.
If we look at pulley system A (Figure 1) and imagine the
effort on the right hand rope pulled down 1m. This means that 1 m of rope must come over
the pulley and so the load rises 1 m. This means that the velocity ratio of our pulley system is
1. However if the pulley is 100 % efficient and the velocity ratio is 1 then the mechanical
advantage is also 1 and that means that the load and effort are the same.
(This particular pulley arrangement does not really make the job
easier except that using your own weight as part of the effort means that there is less effort in
your arm muscles.)
Now in the pulley system with two pulleys things are a little
different.
In pulley system B if the effort on the right hand rope is pulled down 1m then 1 m of
rope must come over the top pulley. This means that EACH of the ropes holding the bottom
pulley must shorten by ½ m and so the load rises ½ m. This means that the velocity ratio of
our pulley system is 2. However if the pulley is 100 % efficient and the velocity ratio is 2 then
the mechanical advantage is also 2 and that means that the load is double the effort. This
means, for example, that you can lift a load of 100 N using an effort of only 50 N.
If a
pulley system is perfectly efficient the mechanical advantage and the velocity ratio are both
equal to the number of pulleys.
No pulley system will be 100% efficient because not
only will there be friction in the axles but the pulleys themselves have weight and also need
energy to be lifted.
See if you understand the idea of pulleys by copying and completing in the following table :
| Number of pulleys | Load (N) | Effort (N) |
| 1 | 12 | |
| 2 | 6 | |
| 3 | 4 | |
| 16 | 8 | |
| 24 | 6 | |
| 100 | 20 | |
| 8 | 640 | |
| 6 | 5 |
