Page 145 - The Complete Rigger’s Apprentice

produce a moment of 450 foot-pounds. That is, Now let’s make a lever that takes a 90-degree
leverage is a matter of weight (50 pounds) times dis- turn at the fulcrum (Figure 5-3). It’s not a seesaw
tance from the fulcrum (9 feet). The children bal- anymore, but the same principles apply. A 50-pound
ance not merely because they weigh the same, but push against the top of the vertical arm, 9 feet from
because they exert the same number of foot-pounds. the fulcrum, is balanced by 50 pounds on the hori-
To prove this, let’s shift the position of the ful- zontal arm, 9 feet from the fulcrum. The same push
crum to the right 6 feet (Figure 5-2). Now the child against the same height vertical arm is balanced by
1
on the left is 15 feet from the fulcrum, which means 100 pounds placed 4 ⁄2feet from the fulcrum: 9 feet
1
she produces a moment of 750 foot-pounds (15 feet 5 50 pounds = 450 foot-pounds, and so does 4 ⁄2 feet
times 50 pounds). The seesaw now extends only 5 100 pounds.
3 feet on the other side of the fulcrum. As you can Hang on, we’re about to start rigging. If you set
see, it takes a 250-pound football player (3 feet up a wire that ran from the end of the horizontal
times 250 pounds) to balance the seesaw. arm to the top of the vertical arm, you would be
exerting leverage on that wire (Figure 5-4). This is
Figure 5-1. The seesaw, a familiar form of lever. how stayed masts work. An important change, other

Figure 5-2. Two hundred-fifty pounds placed 3 feet
from the fulcrum has the same moment as 50 pounds
15 feet from the fulcrum.

Figure 5-4. Take away the hor-
izontal arm, replace it with a
wire, and you’ve started rigging.
The principle of moments still
Figure 5-3. A lever with a applies, but now there’s also a
right-angle turn. compression load on the mast.

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