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Simple Machines

Investigation 3 – PreLab

ZERO-IN

Italicized font represents information to be shared orally or physically completed with the students at this time.

The non-italicized font represents additional information included to support the teacher’s understanding of the content being introduced within the CELL.

ASK WHY

Remind students that as they look about, they probably see half a dozen machines that they don’t recognize as such. Ordinarily, people think of a machine as a complex device-a gasoline engine or a washing machine. They are machines; but so are a hammer, a screwdriver, a bike’s wheel. A machine is any device that helps you to do work.

BRANCH OUT

Remind students that civil engineers design and supervise the construction of society’s infrastructure–bridges, highways, airports, and other projects. In canal systems, these engineers sometimes use inclined planes to pass boats from one level to another.

GET FOCUSED

Inform students that the Investigation is designed to help them to answer the following Focus Questions:

How can simple machines change the force needed to lift a load? In a third-class lever, as the length of the effort arm is changed, the distance over which the effort must be applied also changes. Because the distance over which this force is applied changes, the effort force also changes.

How does the relationship between the fulcrum, effort, and load affect the force needed to lift a load? Changing the position of the fulcrum, effort, and load in relation to one another changes the distance over which a load is lifted and effort is applied. The result is a change in the effort force depending upon the lengths of the effort and load arms and position of the fulcrum, load, and effort.

How does mechanical advantage relate to effort and load forces and the lengths of effort and load arms? Mechanical advantage is equal to 1 when the load and effort force are equal and the length of the load arm and effort arm are equal. As the effort force decreases below that of the load force, mechanical advantage becomes greater than 1. As the effort force increases above that of the load force, mechanical advantage becomes less than 1. As the effort arm becomes longer than the load arm, mechanical advantage becomes greater than 1. As the effort arm becomes shorter than the load arm, mechanical advantage becomes less than 1.

Note: These questions are located in students’ SDRs at the beginning and end of the Investigation.

Note: These are succinct responses to the Focus Questions and are placed here for your reference at this time. Fully developed responses to the Focus Questions can be found on the PostLab page.

GO DEEPER

As a class, read the Background(s) in the Investigation. Have students read the information aloud or silently to themselves. When students have finished, discuss the following concept as a class:

In a third-class lever, the effort is applied between the fulcrum and the load.
Examples of third-class levers include a baseball bat, a car door, and the biceps muscle of the arm.

Note: These concepts are integrated into the Background(s) and are used to deepen students’ comprehension of the big ideas.

LEARN THE LabLearner LINGO

There are no new Key Terms introduced in Investigation 3.

SET FOR SUCCESS

Ask students to summarize what they learned about mechanical advantage in first-class levers.

Note: Student answers may vary. Mechanical advantage can be calculated either by dividing the load force by the effort force, or the effort arm length by the load arm length. In a first-class lever, both effort and load distances change as the position of the fulcrum changes. Therefore, mechanical advantage can be any value greater than zero, depending upon the lengths of the two arms. Although the greatest value for mechanical advantage achieved with a first-class lever in Investigation One was 2, this value was not the greatest value possible with this lever. If the position of the fulcrum had continued to move closer to the load, mechanical advantage would have continued to increase as effort force and load arm length decreased and effort arm length increased.

Ask students to summarize what they learned about mechanical advantage in a second-class lever.

Note: Student answers may vary. In a second-class lever, mechanical advantage will always be greater than 1 because the load is placed between the effort and the fulcrum. Thus, the load arm length will always be shorter than the effort arm length. As a result, the amount of effort force needed to lift the load will always be less than the load force.

Direct students to complete the Recall section in their SDRs. Student answers may vary.

How is energy conserved when using levers?
What is the mechanical advantage afforded by levers?
How does the length of the effort arm affect mechanical advantage?

As a class, encourage students to consider what they know about third-class levers. Ask students to explain the differences between second and third-class levers.

Note: In a second-class lever, the load is positioned between the fulcrum and the effort. In a third-class lever, the effort is positioned between the fulcrum and the load.

Play the video below. Stop to ask students questions or answer students’ questions when necessary. Remind students to follow along with their SDRs and make any notes that they think might be helpful.
After the video, direct students to divide into their lab groups to discuss their strategy for the lab. For example, they may assign certain group members to perform specific functions during the lab.

Note: The purpose of the video is to allow students to anticipate the laboratory experience they will soon encounter. Students should leave this PreLab session with a firm idea of what to expect and how to perform in the lab.

Note: Homework is posted below the video.

HOMEWORK

Tell students that they should review the Investigation in preparation for the Lab.