PULLEYS
LAB 5-2
CC ___
Simulator 5-2
CONCEPT
Pulleys are simple machines that can be used to change the direction of a force, to reduce the
force needed to move a load through a distance or to increase the speed at which the load is
moving. Like other simple machines, pulleys do not change t he amount of work done. However, if
the distance the load moves is decreased in proportion to the distance the force moves, the
required effort is reduced. Pulley systems may contain a single pulley or a combination of fixed
and movable pulleys.
OBJECTIVE
Investigate the efficiencies of different pulley systems and observe how the arrangement of a
pulley system affects the ideal mechanical advantage and efficiency of the system.
MATERIALS
2 single pulleys
2 tall ring stands
2 clamps
2 spring scales
2 double pulleys
1 dowel
1 meterstick
1 set of weights (100 g, 200 g, 300 g, 400 g, 500 g,)
A pulley system with two fixed and two movable pulleys is illustrated in Figure 1. To determine the
efficiency of this machine you must determine the useful work accomplished, known as the work
output, and the work done on the machine, known as the work input.
In general, the work W accomplished when any force F moves an object through a distance d is
W = Fd. If the force is expressed in newtons and the distance in meters, the work is expressed in
joules. The useful work output of a pulley system is the product of the weight, or resistance R,
that is being lifted, and the distance DR that it is raised. Thus
Output = R x DR
(1)
Since the movable pulleys are not part of the load, the fact that they are raised does not add to
the useful work output. This is “wasted” work. Work will also be wasted in overcoming the forces
of friction in the system.
The work input is the product of the force or effort E applied to the pulley system and the distance
DE that E must pull to lift the load to the required height. That is,
Input = E x DE
(2)
The efficiency of the machine is given by
Efficiency 
Output
x100%
Input
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(3)
PROCEDURE
PART I
♦ Determine the weight of R (masses) in newtons
and record its value in table one.
♦ Hang each load in turn from the pulley system and raise it a
distance DS = 0.2m by pulling the spring scale with the least
pull that will keep the load moving upward at a slow constant
speed. Read the force E that you apply to the spring scale
and measure the distance DE that E must pull. (DE is the
length of string pulled out by the spring scale.) Record R, D R ,
E, and DE in table 1.
♦ In each case determine the work output and input from
relations (1) and (2). Then determine the efficiency from
relation (3). Record these values in table 1.
Load
R
(N)
DR
(m)
Effort
E
(N)
DE
(m)
Output
(J)
Input
(J)
Efficiency
(%)
Table 1.
ANALYSIS PART I
1. Make a graph plotting efficiency against load. From it, describe how the efficiency of the pulley
system varies with this set of loads.
2. Why is it necessary in operating the pulley system to raise the load at a slow constant speed?
3. What happens to the energy expended in (a) raising the movable pulleys and (b) overcoming
the friction in this experiment?
4. Does then law of conservation of energy hold in this experiment? Explain.
5. Measure the effort required to keep one of the loads suspended as a given height. How does
this effort compare with the effort required to move the load up at a constant speed? Why?
6. Would you expect a combination of 3 fixed and 3 movable pulleys having the same frictional
characteristics as the pulleys used in this experiment to be more or less efficient than the pulley
used in this experiment? Explain
PULLEYS
LAB 5-2
CC ___
Simulator 5-2
PHYSICSINMOTION
PART II
Assemble and manipulate fixed and moveable pulley systems.
Calculate the efficiencies of different pulley systems.
Inver how the arrangement of a pulley system affects the ideal mechanical advantage and
efficiency of the system.
PROCEDURE
♦ Set up the single fixed pulley system, as shown in pulley arrangement A.
♦ Select a mass that can be measured on the spring scale. Record the value of the mass in
table 2. In this lab the resistance force, F r, of the mass that is lifted is the weight of the mass.
So, Fr = Fg = mg. Determine the weight, in newtons, of the mass to be raised.
♦ Raise the mass by pulling on the spring scale. Measure the distance the mass is raised, d r, in
meters. Record this value in table 1. Calculate the work output, W o, of the pulley system by
multiplying the weight of the mass by the distance it was raised, F rdr. Record this value in
table 3.
♦ Using the spring scale, raise the mass to the same height it was raised in step three and read
the results. Record this information in table 2 as effort force F e of the spring scale. (As you lift
the load with the spring scale, do so slowly as not to introduce the effects of acceleration.
♦ Measure the distance, in meters, through which the force acted to lift the mass to the height it
was raised. Record this value in table 2 as the distance, d e, through which the effort force acts.
Determine the work input, W i, in raising the mass by multiplying the force reading from the
spring scale by the distance through which the force acted Fede. Record the value for the work
input in table 3.
♦ Repeat steps 2 through 5 for a different load.
♦ Repeat steps 2 through 6 for each of the remaining pulley arrangements in figure above. Be
sure to include the mass of the lower pulley(s) as part of the mass raised.
♦ Count the number of lifting strands of string used to support the weight or load for each
arrangement, 1 through 4. Record these values in table 3.
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DATA
Table 2
Pulley
Arrangement
Weight ( Fr )
Mass Raised
Kg
of Mass
(N)
Height ( d r
)
Force ( Fe )
of
Spring Scale
(N)
Mass is
Raised
(m)
Distance ( d e )
Through
Which Force
Acts
(m)
A
B
C
D
Table 3
Pulley
Arrangement
Weight ( Fr )
Mass Raised
Kg
A
B
C
D
of Mass
(N)
Height ( d r
Mass is
Raised
(m)
)
Force ( Fe )
of
Spring Scale
(N)
Distance ( d e )
Through
Which Force
Acts
(m)
PULLEYS
LAB 5-2
CC ___
Simulator 5-2
PHYSICSINMOTION
ANALYSIS
1.
Find the efficiency of each system. Enter the results in table 3. What are some possible
reasons that the efficiency is never 100%.
2.
Calculate the ideal mechanical advantage, IMA, for each arrangement by dividing d e by dr.
Enter the results in table 3. What happens to the effort force, F e, as the mechanical
advantage gets larger?
3.
How does increasing the load affect then ideal mechanical advantage and efficiency of a
pulley system?
4.
How does increasing the number of pulleys affect the ideal mechanical advantage and
efficiency of a pulley system?
5.
The ideal mechanical advantage also may be determined from the number of strands of
string supporting the weight or load. How does the calculated IMA from problem 2 compare
with the number of strands of string you counted for each pulley arrangement?
6.
Explain why the following statement is false: A machine reduces the amount of work you
have to do. What does a machine actually do?
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