Physical Science
The Mechanical Advantage of Pulleys
Pulleys are one type of simple machine, providing a means by which to multiply the affects
of an applied force in moving an object connected to a pulley system. A simple pulley system
may do nothing more than to change the direction of the force being applied. On the other
hand, complex, multi-wheeled pulley systems can provide significant mechanical advantage
for the work of lifting or moving heavy objects.
To lift a heavy object, the mechanical advantage of a pulley system is achieved by
introducing more than one line in the system to deliver upward lifting force in response to a
single force pulling downward on the system. That said, the theoretical mechanical
advantage of a pulley system is equal to the number of lines delivering upward force divided
by the number of lines delivering downward force. The mechanical advantage of the pulley
system shows up in the reduced force necessary to lift a heavy object. In theory, this
applied force would be equal to the weight of the heavy object divided by the theoretical
mechanical advantage of the system.
In a working pulley system, however, there are energy losses due to friction and the weight
of the pulley system itself. Accordingly, the true mechanical advantage of a working pulley
system will prove to be measurably less than the theoretical mechanical advantage of the
system. And, the applied force necessary to lift a heavy object will be measurably greater
than the theoretical applied force previously calculated from the theoretical mechanical
advantage of the system.
At its simplest, the efficiency of a pulley system can be calculated by dividing the
theoretical force necessary to lift a heavy object by the actual force measured
experimentally, stated as a percentage.
Objectives
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To gain knowledge of the operation of pulley systems as tools to assist in the
lifting of heavy objects.
To predict the theoretical mechanical advantage of a pulley system.
To measure the actual theoretical mechanical advantage of a working pulley
system.
To calculate the efficiency of a working pulley system.
Materials
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Ring stands with pendulum clamps, pulley string, and table clamps
Various single and multi-wheeled pulley assemblies
Assorted measured masses
Spring Scales
Physical Science
Experimental Procedure
1. Select a heavy, measured mass weighing in at 500g minimum.
2. Select a pulley wheel assembly, with top and bottom pulley wheels, and construct a
working pulley system.
3. Calculate the theoretical mechanical advantage of the pulley assembly, taking note of
the number of upward lifting lines in the assembly.
4. Calculate the theoretical pulling force necessary to lift the measured mass selected in
step 2.
5. Tie the spring scale into the pulling line and measure the actual pulling force necessary
to lift the measured mass.
6. Calculate the efficiency of your pulley assembly, dividing the theoretical pulling force by
the actual pulling force and multiplying the result by 100%.
7. Repeat steps 2 through 6 for a different pulley wheel assembly, consisting of a
different number of pulley wheels and load bearing lines.
Data Table
Enter your data for the operation of each pulley wheel assembly in the table below.
Heavy, Measured Mass (in grams)
_________________
Heavy, Measured Mass (in Newtons)
_________________
Pulley
Wheel
Assembly
Trial
Number of
Upward
Lifting
Lines
Theoretical
Mechanical
Advantage
Theoretical
Pulling
Force in
Newtons
Actual
Pulling
Force in
Newtons
Experimental
Mechanical
Advantage
Efficiency
Conclusions
Write a short essay, explaining in your own words how pulley systems provide mechanical
advantage in doing work to lift a heavy object. Explain in your own words how it is not
possible for a pulley system to work at 100% efficiency. Finally, discuss how work input on a
pulley system compares to the work output of the system.