Levers have a lot of Class

Sorting Mechanisms

Procedure: Students are introduced to the terms “fulcrum”, “effort,” and “load,” and asked to identify these points on several of their devices. Using the Sorting Mechanisms Worksheet they then sort the mechanisms from “What is a Mechanism?” according to the arrangement of the effort, fulcrum and load: effort-fulcrum-load (1st class lever); effort-load-fulcrum (2nd class lever); or load-effort-fulcrum (3rd class lever).

“Input” and “output” are general terms that could be used with any system. A special language is used in describing the most basic type of mechanism, the lever. The input is often called the effort, and the output is called the load. As with the terms “input” and “output,” both the “effort” and “load” could describe either a force or a location.

In addition to the effort and the load points, every lever has a pivot that allows the rest of the device to rotate. In the language of levers, the pivot is called the fulcrum. Several important characteristics of lever operation depend on the arrangement of the three elements: fulcrum, effort and load. There are three different possibilities, labeled as first-, second- and third-class levers below.

Lever classes

Sorting Mechanisms (continued)

To see what class a particular lever falls into, identify the fulcrum, effort and load, and compare their arrangement with the diagrams above. The diagrams below show how this can be done for some examples from each class.
1st class lever examples
2nd class lever examples
3rd class lever examples

Compound Levers

Procedure: A compound lever consists of one lever operating another. Students should have encountered some compound levers during the sorting activity, such as the nail clipper. Using the Compound Levers Worksheet they should identify each of the nail clipper’s levers and find the effort, fulcrum and load for each one. Then ask each group to select one compound lever; make a sketch of it showing each of the simple levers; and label the effort, fulcrum and load on each one. Based on this drawing, they should be able to tell the class of each lever. Then ask them to represent the compound lever in their sorting categories.

Nail clipper, showing that the load of the handle is the effort of the jaw The nail clipper is an example of a compound lever. The image to the right shows the two levers that make it up: the handle and the upper jaw. Notice that the load of the handle is also the effort of the jaw; in other words, the handle is in series with the jaw. From the drawing, it should also be clear that the handle is second-class, while the jaw is third-class.
Assorted compound levers
Some other compound levers are shown to the left. Clockwise starting at the top, these include:
    • a pair of vise grips (1st class lever as input to 1st class lever)
    • a pizza-tray holder (same as vise grips)
    • a grapefruit sectioner (1st class lever as input to 3rd class lever)
    • a tea-bag strainer (reverse of grapefruit sectioner)

    • a pair of garden shears (2nd class lever as input to 1st class lever)
    • a pair of tin snips (see Extreme Scissors for an extended discussion)

Venn diagram showing classes of levers An interesting issue is how to represent these compound levers in sorting according to first, second- or third-class. One solution is to use a Venn diagram.

Design Challenge 1

Procedure: Demonstrate sample mystery mechanisms A & B shown below. Challenge students to create these two mechanisms, using paper fasteners and cardboard.

Mystery mechanism challenge A Mystery mechanism challenge B

To help students solve these challenges, ask them to examine the directions of the input and output in each case. Do they travel in the same or opposite directions? Then suggest that they look at the first-, second- and third-class levers they have sorted. In which of the classes do the input and output go in the same direction? Which class makes them go opposite ways? If the input and output are in the same direction, which class has the output at the end? Based on this analysis, students should conclude that the mystery mechanism A must contain a third-class lever, while B is based on a first-class lever. Complete solutions are shown below

Solution to design challenge A Solution to design challenge B
Key to the solution graphics

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