simple pulley system
The simplest pulley system consists of two groups that are
identical. The upper group is the fixed one. The string goes
over the upper group’s first pulley and then comes down to
the lower group, which is moving, and wraps around the
lower group’s first pulley. Then it comes back to the upper
group and is tied to it, or it can be wrapped around a second
pulley and repeat the arrangement between the first two
pulleys. This means that the string can’t be tied directly to
the hook after it comes from the first upper pulley, which
is the main difference between the simplest pulley system and
the lack of any such system, as Figures 6-16 and 6-17 show.
As you can see in Figure 6-17, the simplest pulley system
has two sections of string connecting the two groups. This
means that in order to lift the load, we have to wind up
twice as much string as without this system, but using only
half the force. The weight of the load is reduced twice at the
cost of more string to be wound. There’s no free lunch: We
are trading time for work, having to do less work but over a
longer period of time.
But let’s consider what happens if we add another
section of string between the two groups. We will need one
more pulley to prevent the sections from getting tangled up
with one another, as shown in Figure 6-18.
As you see, the string is now tied to the lower group,
but only after it goes through three pulleys: two upper ones
and one lower one. Three sections of string connect the
two groups, granting a mechanical advantage of 3. We
need to wind up three times as much string but use only
one third of the force. By now you have probably guessed
that the number of sections of string connecting the groups
determines the mechanical advantage they realize.