Page 26 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 1 Introduction to Mechanisms and Machines 7

Can you think of some other first FIGURE 1-5 The wheelbarrow as a second
class levers? class lever

Second Class Levers
In a second class lever, the output is
located between the input and the
fulcrum. The classic example of this is the
wheelbarrow. As you can see in Figure 1-5,
the stuff in the wheelbarrow is the output
or load, and we use the handles as the
input.
We can use the same equation as for first
class levers to figure out the balance of
forces. Let’s say we have a 50 lb load (F ) of
2
bags of gold in the wheelbarrow, and the distance from where the bags of gold are to
the wheel is 1 ft (d ). If the handles are 5 ft long from the grip to the wheel (d ), how
2 1
hard do we need to pull up to lift the bags of gold? Let’s put what we know into our
equation:

F × 5 ft = 50 lbs × 1 ft
1
So in order to lift the bags of gold, we must pull up on the handles with at least 10
lbs of force (F ). See that? We can move 50 lbs of bags of gold with only 10 lbs of pull
1
force, for another 5:1 mechanical advantage—the same as we saw with Fido and
Fluffy on the seesaw.

Another household item that uses a second class lever is a bottle opener. In Figure 1-6,
you can see the input, fulcrum, and output identified. The handle of the bottle opener
goes through a lot of travel to get the cap of the bottle off, but the force at the lip of
the bottle cap is relatively high. A nutcracker is another example of a second class
lever. Can you think of any other second class levers?

First and second class levers are force multipliers, which means they have good
mechanical advantage. The trade-off in both cases is that the input, or effort, must
move a greater distance than the output, or load.