Begin the Investigation by reviewing the experiments students completed in Investigation One and Two, and conclusions they have drawn about the concept of force. 

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The following questions may help involve students in the discussion of the concepts:

How would you describe the concept of work? Work is a measure of the magnitude of a force applied to an object and the distance over which it is applied.  Work varies with the amount of force applied and the distance over which it is applied.

What two quantities do you need to calculate work? Distance and force.

How would you explain the relationship between force and work, and distance and work? Students discovered that as the distance a load is lifted increases, the amount of work done increases.  They also discovered that as the mass of a load increases, the amount of force exerted to lift the load increases, which in turn increases the amount of work done to lift the load.

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Note: Data for this problem are different than in the Student Data Record and Teacher’s Manual

Assess students’ current understanding of the concept of work by giving them a problem to solve.

1. Tell students that another student, Cindy, has injured her back.  Cindy’s doctor tells her not to do more than 100 joules of work when lifting an object.

2. Divide students into pairs or cooperative groups to work through the problem.

3. Lead students through the problem by posing the following questions and information, stopping after each to allow groups to discuss and present their ideas.

a. What does Cindy need to know so she doesn’t do more than 100 J of work? If Cindy is to follow her doctor’s orders, she must know the distance she would lift or move a load and the amount of force she would exert to lift the load.

b. Cindy wants to bake a turkey in the oven, but she must be careful not to do more than 100 J of work when she lifts the turkey out of the oven.  The distance she will need to lift the turkey, from the oven to the counter in her kitchen, is 50 cm.

c. Why is it important to know the distance Cindy must lift the turkey? As the distance Cindy must lift the turkey increases, the amount of work she does will increase.

d. The roasting pan and the turkey have a mass of 15.3 kg (15,300 g) and Cindy would exert a force of 150.0 N to lift them.  How much work will Cindy do if she lifted the roasting pan and the turkey from the oven to the counter? 150.0 N × 0.5 m = 75.0 J

e. Would Cindy be able to lift the turkey without hurting herself?  Why or why not? Yes. Lifting the turkey and roasting pan would require 75 joules of work.  This is less than the 100 joules suggested by her doctor.

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Ask students:  Having studied work and the factors that contribute to work, what could you do to make work easier? Student answers will vary.  Students may suggest that decreasing the force applied to a load or the distance to lift a load would decrease the work done to lift a load.  A decrease in work may make work feel easier.  However, students may also suggest that work may be easier by using a machine.

Initiate a discussion with students to introduce simple machines.  Tell students they can refer to their Scientist’s Glossary as you ask the following questions.

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Where have you heard the term machine before?  What do you think machines do? Student answers will vary.  Students may refer to cars, washing machines, or refrigerators as examples of machines.  They may describe the specific functions of each machine or suggest that machines make work easier or decrease work.

Affirm students’ examples of machines.  Explain to students that one type of machine is a simple machine and that students will experiment with simple machines in Investigations Three, Four, and Five.  Tell students that, just like force and work, a simple machine has a very specific scientific definition.

Using the Scientist’s Glossary as a reference, introduce a simple machine as a device that makes work feel easier.  Explain that there are six simple machines: the pulley, lever, wheel and axle, screw, wedge, and inclined plane.

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Tell students that this Investigation will use one simple machine, a fixed pulley. 

Ask Students: Have you ever seen a fixed pulley?  Could you describe a fixed pulley? What is a fixed pulley? Student answers will vary.  A fixed pulley is a simple machine that is attached to a surface and uses a stationary grooved wheel through which a rope, belt, chain, etc. runs.

Discuss with the class the definitions for the terms effort, effort arm, load, and load arm.  Draw an example of a simple fixed pulley on the board with a load attached and label the effort arm, load arm, and load.  Explain the operation of a fixed pulley with the class, including the four terms in your discussion.

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Ask students: Can you think of an example of a fixed pulley in your everyday lives?  Student answers may vary.  Examples are that pulleys are used with cranes, flagpoles to raise and lower the flag, elevators, to raise and lower window blinds, some curtains use pulleys to open and close them, etc.  Use the examples to encourage students to think about how a pulley can be used.

In this slide, students may try to identify the pulleys in three examples:

Top Left: Elemator pulley

Bottom Left: Weight machine pulleys

Right: Crane pulleys

Explain to students they will examine how the pulley affects the work and force exerted on a load.  As they perform their experiments, they should think about the following two questions:

• Does a pulley change the amount of work done to lift a load?
• Do simple machines decrease the amount of work done to move a load?