Page 23 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

4 Making Things Move

FIGURE 1-1 The classic playground seesaw is an example of a first class lever.

3. The two things can have different weights, and the lighter one must be farther
from the fulcrum in order to balance. If you’ve ever been on a seesaw with
someone heavier than you, you’ve probably done this without thinking about it.
If you were the lighter one, you backed up as far as you could to the edge of
the seesaw, and your heavier friend probably scooted in toward the pivot point.

In order to apply these balance rules to machines, let’s replace the word thing with
force. But first, meet Fido and Fluffy.
Fido is a big dog. Fluffy is a small cat. Because their names both start with F, I’ll use F
1
for Fido and F for Fluffy when I abbreviate them. Fido is heavier, so his arrow (F ) on
2 1
the left side of Figure 1-2 is bigger. He is sitting at a certain distance (d ) from the
1
fulcrum. Similarly, Fluffy (F ) is at a distance d from the fulcrum on the right side. In
2 2
order to balance the seesaw, F times d must equal F times d :
1 1 2 2
F × d = F × d
1 1 2 2
You can see from Figure 1-2 and the equation that if F = F , and d = d , then the
1 2 1 2
seesaw will look like Figure 1-1 and balance. But if Fido (F ) is a 50 pound (lb) dog,
1
and Fluffy (F ) is a 10 lb cat, then they must adjust their distances to the fulcrum in
2
order to balance. Let’s say that Fido is 3 feet (ft) away from the fulcrum (d = 3 ft).
1
How far away from the fulcrum does Fluffy need to be to balance? Now our equation
looks like this:

50 lbs × 3 ft = 10 lbs × d 2