Name
Class
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Chapter 15
Date ----
Laboratory Investigation
Work, Power, and Simple Machines
40
Pulleys As Simple Machines
Background Information
Pulleys are simple machines that are used in different ways to lift objects. The simplest
kind of pulley is a grooved wheel around which a rope is pulled.
Pulleys can be used to change the direction of an applied force. For example, a pulley
attached, or fixed, to the top of a flagpole allows you to raise the flag up by pulling down.
A combination of fixed and movable pulleys is called a pulley system, or block-and-tackle.
A pulley system is used to multiply effort force in lifting heavy objects. Pulley systems are
commonly seen around construction sites.
In this investigation you will see how different pulleys are used and determine the
mechanical advantage of each.
Problem
How are pulleys used to raise objects? How is the mechanical advantage of a pulley or
pulley system determined?
Materials (per group)
2 single pulleys
2 double tandem pulleys
1 m nylon fishing line
ring stand and large ring
spring balance
l-kg mass, or larger
Procedure
1. Find the resistance force of the mass you are using by attachin~ it directly to the spring
balance. Record this resistance in the Data Table as the resistance for all of the pulley
arrangements.
2. Set up a single fixed pulley as shown in Figure 1. Pull down on the spring balance to lift
the mass. The reading on the balance shows the amount of effort needed to lift the
resistance. Record this number in the Data Table.
Figure 1
© Prentice-Hall,
Inc.
177
3. Set up a single movable pulley as shown in Figure 2. Lift the mass by pulling up on the
spring scale. The reading on the balance shows the amount of effort needed to lift the
resistance. Record this number in the Data Table.
Figure 2
4. Set up the pulley system shown in Figures 3, 4, and 5. For each pulley system, measure
the amount of force needed to lift the resistance and record it in the Data Table.
Single Fixed, Single Movable
Double Fixed, Single Movable
Double Fixed, Double Movable
Figure 4
5. Calculate the mechanical advantage for each pulley and record these numbers in the
Data Table.
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Name
Class
Date
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Observations
DATA TABLE
Pulley Arrangements
Resistance (R)
Effort (E)
Mechanical Advantage
(R + E)
Single fixed
Single movable
Single fixed and single movable
Double fixed and single movable
Double fixed and double movable
Analysis and Conclusions
1. Was there a difference in the mechanical advantages you calculated for the single fixed
pulley and the single movable pulley?
Explain your answer.
2. As you added pulleys to the system, what happened to the amount of effort force needed
to raise the mass?
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3. How does the type of pulley or pulley system affect the mechanical advantage?
Critical Thinking and Application
1. If a simple machine has a mechanical advantage of 1, effort force is not multiplied.
Which type of pulley has a mechanical advantage of I?
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What is the practical use of this pulley?
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© Prentice-Hall, Inc.
179
2. To determine the mechanical advantage of a pulley or pulley system without arithmetic
calculations, it is possible to simply count the number of sections of rope that support the
resistance mass. The end section, to which the balance is attached, counts as a supporting
section of rope only when pulled up. Using Figures 1 through 5, determine the number of
supporting rope sections for each type of pulley.
a. Figure 1
b. Figure 2
c. Figure 3
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d. Figure 4
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e. Figure 5
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3. Do the values obtained in question 2 agree with the calculated mechanical advantage in
the Data Table? ___
Explain your answer.
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4. Using two double pulleys, draw an arrangement of the pulleys that would give you a
mechanical advantage of 5.
5. When using any simple machine, you never "get something for nothing." Although the
amount of effort force needed to lift a mass is reduced in a pulley system, something else
is increased. What must be increased as the amount of effort force is decreased?
6. Explain your answer to question 5 in terms of work input and work output.
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