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 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.
(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.
| Pulleys | Load (N) | Effort (N) | Pulleys | Load (N) | Effort (N) | |
| 1 | 12 | 24 | 6 | |||
| 2 | 6 | 100 | 20 | |||
| 3 | 4 | 8 | 640 | |||
| 16 | 8 | 6 | 5 |