Teacher Portal:
Work and Simple Machines
Investigation 3 – Lab
BE PREPARED
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Teacher Preparation
1. Organize the required materials at a distribution point.
2. Cut one (1) 1 m length of string for each group. (LabLearner Closet: Clamp a pulley to one of the tables or desks that will be used by the students. Measure and cut a piece of string that when hung from the pulley is 10 cm from the floor at one end and is 20 – 40 cm from the floor at the other end.)
3. For each piece of string, make a loop at one end that will be
used to insert the hook of the spring scale. Make another loop
at the other end and insert the paper clip so that the white
bucket can be attached.
4. Separate the class into five groups.
Instruction
Instruct each student group to obtain the following necessary materials from the distribution point: one (1) fixed pulley, one (1) spring scale, one (1) white plastic bucket, one (1) meter stick, one (1) metal cubes, three (3) large (12 g) bears, three (3) medium (8 g) bears, three (3) small (4 g) bears, ten (10) gram cubes, and one (1) piece of string with a loop at one end and a paper clip at the other end.
Explain that, in this investigation, students will explore a simple machine, the fixed pulley. Students will assemble a pulley system to explore how this simple machine affects work. The passages that follow are provided to assist your direction of the students through this part of the investigation.
a. Obtain a fixed pulley and clamp it securely to the edge of the table by tightening the screw (Figure 3.3a).
b. Using the pre-cut length of string, students should thread the end of the string with the paper clip through the groove of the pulley’s wheel and allow it to hang from the back of the pulley (Figure 3.3b). Adjust the string until approximately equal lengths hang from each side of the pulley (Figure 3.3c).
c. Place the following into the white bucket: 10 gram cubes, 1 metal cube, 3 large bears, 3 medium bears, and 3 small bears.
d. Insert the paper clip into the hole in the handle of the bucket containing the bears, metal cube, and gram cubes (Figure 3.4a).
e. Insert the hook of the spring scale into the loop at the other end of the string. The spring scale will hang upside down (Figure 3.4b).
f. Test the pulley system by inserting an index finger into the handle of the spring scale and pushing down. The load attached to the other end of the string should lift off the floor (Figure 3.4c).
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- To begin their investigation, students will sketch and identify the major components of the fixed pulley they assembled including the effort arm, load arm, and load. What are the parts of a pulley?
GET FOCUSED
Students will continue to explore the use of the new tool from their Procedural Toolbox, the spring scale.
INVESTIGATE
Trial 1
In this trial, students will determine the work done to lift a load without a simple machine. They will use the spring scale to obtain a quantitative measurement for the force exerted to lift the load without a pulley. After lifting the load a distance of 20 cm, students will use their measurements to calculate the work done to lift the load. Students will be asked a question that is the main focus of this part of the investigation. They should think about this question as they perform the steps of the experiment:
How much work is done to lift the load without the pulley?
Trial 2
In this trial, students will determine the work done to lift a load using a pulley. They will use the spring scale to obtain a quantitative measurement for the force exerted to lift a load when using a pulley. After lifting the load a distance of 20 cm, they will use their measurements to calculate the work done to lift the load. They will be asked to think about the following question as they perform the steps of the experiment:
How much work is done to lift the load when lifting with the pulley?
Trial 3 and 4
Trials 3 and 4 have been designed to provide students additional opportunities to compare the work done to lift a load with and without the use of a pulley. Although students compared the work done to lift the load when using and when not using a pulley in Trials 1 and 2, students may believe that the work was similar when lifting only that load or a load of that particular mass. As students complete Trials 3 and 4, they will vary the mass of the load and compare the work done to lift the load both with and without the pulley. In doing so, they should conclude that the work done to lift a load is the same when lifting with and without a pulley, regardless of the mass of the load. Before students begin their experiment, they will consider the following questions:
You discovered that the work done to lift a load was the same with and without a pulley. Is this always the case?
Is this the case even if a load of a different mass is lifted?
Students should follow the same procedures when lifting the loads and using the pulley as in Trials 1 and 2. If necessary, provide extra support for students during the process by guiding them through the steps.
Trials 5 and 6
Trials 5 and 6 have been designed to provide students additional opportunities to compare the work done to lift a load with and without the use of a pulley. In these trials, the mass of the load remains constant while students vary the distance the load is lifted. By doing so, students should conclude that the work done to lift a load is the same when lifting with and without a pulley, regardless of the distance the load is lifted. Before students begin their experiment, they will consider the following questions:
You discovered the work done to lift a load was the same with and without a pulley. Is this always the case?
Is this the case even if the load is lifted a different distance?
Students should follow the same procedures when lifting the loads and using the pulley as in previous Trials. If necessary, provide extra support for students during the process by guiding them through the steps.
KEYS
CLEAN UP
Let students know your expectations for clean-up. Ask them to clean up.