SIMPLE MACHINES, FIXED AND MOVEABLE PULLEYS
RST-LAB-013 REV B
A) OBJECTIVE: To compare the mechanical advantage (MA) of fixed and moveable pulley systems.
B) BACKGROUND Referring to the handout, a pulley is an example of a simple machine. A simple machine is a device that
performs work by converting forces and distances. A simple machine has an attribute called a Mechanical Advantage (MA). The MA
of a pulley system can be determined by the formula:
C) PROCEDURE
Step 1: Build the Fixed Pulley Assembly per instructions
Step 2: Add 8 to 10 washers to the load and weigh the assembly per Figure 1. Record the weight in the
Data Table under “Force Needed to Lift Load Without Pulley”.
Step 3: Attach the load (weight) to the Fixed Pulley Assembly. Pull on the string to verify the load
moves up and down smoothly.
Figure 1
Step 4: Using the spring scale, pull down on the string and raise the load per Figure 2. Measure the force shown on the spring
scale and record in the Data Table under “Force Needed to Lift Load With Pulley”.
Step 5: Calculate the Mechanical Advantage of the pulley system using the formula above.
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SIMPLE MACHINES, FIXED AND MOVEABLE PULLEYS
RST-LAB-013 REV B
Step 6: Reconfigure to the Moveable Pulley Assembly as shown in the instructions. Repeat Steps 3, 4 and 5 as shown in Figure 3.
Figure 2
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Figure 3
Figure 4
SIMPLE MACHINES, FIXED AND MOVEABLE PULLEYS
RST-LAB-013 REV B
Note: We want to compare the amount of force required to lift the weight between the Fixed and Moveable Pulley systems. Use the
same weight for both measurements- Do not change the amount of weight of the load. Enter the force required to lift the load with
the pulley in the Data Table under “Force Needed to Lift Load With Pulley”.
D) DATA TABLE
Force Needed to Lift
Load Without Pulleys
Force Needed to Lift
Load With Pulleys
Mechanical
Advantage
Fixed Pulley
Movable Pulley
E) QUESTIONS
1) What is the Mechanical Advantage of the Fixed Pulley? What is the purpose of the Fixed Pulley?
2) What is the Mechanical Advantage of the Moveable Pulley? What is the purpose of the Moveable Pulley?
3) How could you increase the Mechanical Advantage of a pulley system?
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SIMPLE MACHINES, FIXED AND MOVEABLE PULLEYS
RST-LAB-013 REV B
PULLEYS: WHAT YOU NEED TO KNOW
A pulley is a simple machine with a groove on its outside edge in which a cable (string) runs,
At one end of the cable is the LOAD (also called the RESISTANCE or the weight you want to lift)
At the other end of the cable is the EFFORT (the force you apply)
There are two types of pulleys: FIXED PULLEYS and MOVEABLE PULLEYS
FIXED PULLEYS change the direction of the EFFORT (Mechanical Advantage of 1)
MOVEABLE PULLEYS decrease the EFFORT by converting force over a greater distance (ie trades force for distance)
Mechanical Advantage = Force required WITHOUT pulleys / Force required WITH pulleys
FORCE
FORCE = LOAD
2•FORCE = LOAD
FORCE = LOAD / 2
EFFORT = RESISTANCE
MA = 1
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EFFORT = RESISTANCE / 2
MA = 2
Rev B