In this third and final Investigation in the CELL Simple Machines, we will discuss third-class levers. We also include a brief review of first and second-class levers and a final overview of the six traditional classes of simple machines.

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• This slide provides students with a final reminder of the six types of simple machines. At this point, we have spoken in detail about pulleys and levers and have at least discussed the mechanical advantage for the other four simple machines.

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• This slide has been seen before. It shows various types of levers. The hammer and pliers are both first-class levers, as is the preying bar in the center illustration. The bottle opener and nutcracker are second-class levers.

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Note: This slide should be used to review the fundamental differences between first, second, and third-class levers. Further detail is shown for each lever type in the following slides.

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• First-class levers. Remember that in first-class levers, the fulcrum is located between the effort and the load.

Note: At this point, students should be able to identify all three of these features of first-class levers (fulcrum, effort, and load).

Note: In addition, students should be able to identify the length of the effort arm and load arm in relation to the fulcrum as well as the load force and effort force. Given this information, students should now be able to apply the formulas for mechanical advantage due to force (MA_f) and mechanical advantage due to distance (MA_d).

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• Second-class levers. Remember that in second-class levers, the fulcrum is located at one end and the load is situated between the fulcrum and the effort. The classic example of a second-class lever is a wheelbarrow.

Note: Students should easily be able to identify the length of the effort arm and load arm as well as the load force and effort force. Given this information, students should also be able to apply the formulas for mechanical advantage due to force (MA_f) and mechanical advantage due to distance (MA_d).

Note: Students should also notice that the formula for both mechanical advantage due to force (MA_f) and mechanical advantage due to distance (MA_d) for second-class levers are exactly the same as for first-class levers. They will soon see that the same formulas apply to third-class levers as well.

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• In this slide, we introduce the third-class lever. In third-class levers, the fulcrum is at one end as in second-class levers, but in this case, the effort is applied between the load (which is typically at the opposite end from the fulcrum) and the fulcrum.

• Examples of third-class levers shown here are forceps, tongs, a fishing rod, and a broom. Other examples of third-class levers include a hockey stick, baseball bat, and chopsticks.

• The experimental setup for the first part of Investigation 3 lab is shown at the upper right. It is shown in greater detail in the following slide.

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• This slide not only illustrates the experimental setup for the third-class lever for Trials 1-4 in Investigation 3 lab but also presents the data table from the Student Data Record and clearly shows where the data for each field of the Table is derived.

• Notice that the formula for both mechanical advantage due to force (MA_f) and mechanical advantage due to distance (MA_d) for third-class levers are exactly the same as for first and second-class levers.

Note: The teacher may wish to review this slide once again as a component of the PreLab.

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Note: This slide is animated. The teacher may wish to practice the steps prior to presentation to the class.

• NO ANIMATION: This slide shows the biceps muscle as a third-class lever. The important bone in this lever is the radius of the forearm. The fulcrum of this lever is the elbow. The biceps muscle attaches to the shaft of the radius, not too far for the elbow. Contraction of the biceps causes a shortening of the muscle and also an upward force on the radius. The net result of the force and movement is that the forearm is raised into a position closer and more parallel to the humerus bone of the upper arm.

• FIRST ANIMATION: The first animation causes the outer drawing of bone and muscle to fade and reveals the analogous components of the third-class lever that will be used in Investigation 3 lab.

• SECOND ANIMATION: With the second animation, the components of the underlying lever model are labeled. We will focus on this model in the following slide.

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• This final slide not only illustrates the experimental setup for the third-class lever for Trials 5-7 in Investigation 3 lab but also presents the data table from the Student Data Record and clearly shows where the data for each field of the Table is derived.

• Notice again that the formula for both mechanical advantage due to force (MA_f) and mechanical advantage due to distance (MA_d) for a third-class lever are exactly the same as for first and second-class levers.

Note: The teacher may wish to review this slide once again as a component of the PreLab.

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