Download Mechanical Advantage and Pulleys

2.4
CONDUCT AN INVESTIGATION
Mechanical Advantage
and Pulleys
Pulley systems are an important part of many machines. In this
Investigation, you will be testing factors that might influence the
actual mechanical advantage of pulley systems.
A single fixed pulley, like the one in Figure 1(a), has an ideal
mechanical advantage of 1 since the effort force (120 N) is equal to
the load force (120 N). However, in the single moveable pulley
(Figure 1(b)), half of the load is supported by the rope attached to
the ceiling. The other half is supported by the free end of the rope,
where the effort force is applied. Therefore, the effort force needed
to move the load is only one-half the load force, or 60 N. The
mechanical advantage of this pulley is 2. A simple way to determine
the ideal mechanical advantage of a pulley system is to count the
number of lengths of rope between pulleys that support the load.
In Figure 1(a), only one segment of rope supports the load.
Therefore, the mechanical advantage is 1. In Figure 1(b), two
segments of rope support the load, so the mechanical advantage is 2.
SKILLS MENU
Questioning
Hypothesizing
Predicting
Planning
Controlling
Variables
load
force
120 N
load
force
120 N
(a)
(b)
Figure 1
Equipment and Materials
How does the number of pulleys used in a pulley
system affect mechanical advantage?
• spring scale
• single pulley
• double pulley
• triple pulley
SKILLS HANDBOOK
2.B.3.
Make and record a hypothesis for the testable
question. Your hypothesis should include a
prediction and reasons for your prediction.
input
force
60 N
input
force
120 N
Testable Question
Hypothesis/Prediction
Performing
Observing
Analyzing
Evaluating
Communicating
• standard mass or
washers
• metre stick
• string
Experimental Design
Mechanical advantage is the dependent variable
you will be studying in this investigation. The
independent variable you will be testing is the
number of pulleys in a system.
spring scale
single pulley
double pulley
triple pulley
standard mass/
washers
metre stick
string
44 Chapter 2 • Getting to Work
Sci8_UnitA_Chap2.indd 44
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SKILLS HANDBOOK
Procedure
4.C.1., 6.A.
1. Create an observation table as set out in
Table 1. In your table, allow enough space for
small sketches in the first column.
Table 1
Number of pulleys used
Sketch of
pulley system
Number
of pulleys
1
1
Type of
Output
Input
MA
pulleys
force (N) force (N)
single fixed
single
moveable
2. Lift the mass with the newton spring scale.
Record this value in the Output force column
for each of your steps.
3. Suspend a single fixed pulley between two
desks or chairs and use it to lift the mass.
4. Attach the free end of the string to the spring
scale and measure the force required to lift
the mass (Figure 2). Record this in the Input
force column.
Figure 2
Step 4
5. Complete the MA column.
6. Repeat steps 3 to 5 for a system that uses a
single moveable pulley.
7. Repeat steps 3 to 5 for a system that uses a
single fixed and a single moveable pulley.
8. Repeat steps 3 to 5 for a system that uses a
single fixed and two moveable pulleys.
NEL
Sci8_UnitA_Chap2.indd 45
Analyze and Evaluate
(a) Restate the independent and the dependent
variables for your experiment.
(b) Review the Testable Question and your
hypothesis. Do your observations support
your prediction? Use your observations to
state your evidence.
(c) How could you have improved your
experiment? Give reasons for your
suggestions.
(d) What did you find difficult about working
with these materials? What did you find easy?
Apply and Extend
SKILLS HANDBOOK
2.B.3., 2.B.4.
(e) In this Investigation, you analyzed the effect
of changing the number of pulleys on the
mechanical advantage of the pulley system.
However, the mechanical advantage of pulley
systems may be affected by the arrangement
of pulleys as well. For example, a pulley
system may include a single fixed pulley
attached to a single movable pulley, as in
Figure 6 of Section 2.3.
i)
Design an investigation in which you
explore the testable question “How does
the arrangement of pulleys in a pulley
system affect the pulley’s mechanical
advantage?”
ii)
Make a hypothesis to address this
question and describe a procedure
for performing your investigation. Be
sure that your pulley arrangements are
different from those used earlier in this
Investigation.
iii) With your teacher’s permission, carry
out your investigation and make
conclusions. You may wish to obtain
more information on pulley systems by
visiting the Nelson Science website.
Go to Nelson Science
2.4 Conduct an Investigation 45
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