Pulley Systems
Introduction:
The lever is only one simple machine that makes a job easier for you by multiplying your
effort force, multiplying distance/speed, or changing the direction of a force. The pulley is
another useful device. What is a pulley? It is nothing more than a grooved wheel that is free to
turn so that a rope can be passed over it. This simple machine allows a person to stand on the
ground and lift things far above her head. In addition, a pulley will do all three things that a
lever can do. The way in which a pulley is arranged will decide how it will make a job easier for
us.
Machines like the pulley help us to do work. Work has several meanings. Sometimes
you have “homework”. When a person goes to his job, he will say he is “going to work”. In
science, work has a special meaning. If a force causes an object to move a distance, then work
has been done. Work is equal to the product of force and the distance moved in the direction of
the force. Thus,
Work = Force x distance
When machines are used to help us do work, two terms are used.
Work input (Win = Fe x de) is the amount of work done by you in using the machine.
Work output (Wout= Fr x dr )is the amount of work actually done in moving the object.
LAB # 5-2
Pulley Systems
MATERIALS: Pulleys, strings, ring stands, weights, spring scales
PURPOSE: In this investigation you will arrange a pulley and pulley systems in different ways
to discover the advantage of each arrangement. After completing this investigation you will
be able to:
 recognize the advantages and disadvantages of various pulley arrangements.
 construct pulley systems to increase force and/or change direction.
 calculate the mechanical advantage of a pulley system.
HYPOTHESIS: none- this is an informational lab
PROCEDURES:
PROCEDURES: Part 1 – Single Fixed Pulley
1. Attach a pulley to a ring stand with a piece of string as shown in figure 1. Because the
pulley is attached to the ring it is called a fixed pulley or a stationary pulley.
2. Find a spring scale. The spring scale is designed to be used with the hook hanging
down. You will need to calibrate the scale by holding it by the ring with nothing
hanging off of the hook. Now move the sliding tab on top of the scale until the
zero mark lines up with the scale pointer.
3. Thread a piece of string measuring about 1 meter over the pulley. Make small
loops on each end. Hang the resistance force – a 100 g (1 N) mass to one loop and a
spring scale to the loop on the other end of the string.
4. For Trial 1 pull the spring scale downward and take a reading. Record you reading to
the nearest .5 N in Data Table 1 as the Effort Force (Fe).
47
Figure 1
5. To determine the effort distance (de) for Trial 1, measure the vertical distance you move
down as the resistance moves up .10 m. Record de in Data Table 1.
6. For Trial 2 add another 100 g mass to give a resistance force (Fr) of approximately 2 N.
Measure the effort force and effort distance as you did in Trial 1 and record your readings in
Data Table 1. Note how the resistance distance changes with each trial.
7. For Trial 3, add another 100 g to give a 3 N resistance force. Again, measure the effort force
and effort distance and record your readings in Data Table 1.
8. Use the data from Data Table 1 to calculate the work input and work output of the single
fixed pulley. Record your answers in Data Table 2. Use the standard Win and Wout formulae
found in the introduction.
9. Calculate the mechanical advantage of the single fixed pulley for each trial and record this in
Data Table 2. Use the formula Ideal MA = de / dr.
DATA TABLE 1 –Single Fixed Pulley
Effort
Force
(Fe)
Effort
Distance
(de)
DATA TABLE 2 – Single Fixed
Pulley: Work & MA
Work
Work
Trial
output
Input
(Wout)
(Win)
Trial
Resistance
Force
(Fr)
Resistance
Distance
(dr)
1
1N
.10 m
_____N
_____m
1
_______Nm
_______Nm
2
2N
.15 m
_____N
_____m
2
_______Nm
_______Nm
3
3N
.20 m
_____N
_____m
3
_______Nm
_______Nm
QUESTIONS: Part 1-Single Fixed Pulley
1. How did the effort force compare to the resistance force in each trial?
a. Fe < Fr
b. Fe > Fr
c. Fe = Fr
2. How did work input compare to work output for the 3 trials?
a. Win < Wout
b. Win > Wout
c. Win = Wout
3. Compare the mechanical advantage in the 3 trials.
a. MA increased
b. MA decreased
c. MA remained equal
4. What kind of help or advantage does a single fixed pulley provide? (choose one)
a. Multiplies effort force
b. multiplies distance/speed c. changes direction
5. Name 3 places you might find a single fixed pulley (other than science class).
48
MA
PROCEDURE Part 2 – Single Moveable Pulley
1.
Arrange the pulley as shown in Figure 2. Notice that the pulley is free to move
and is not attached to the ring stand. Therefore, it is called a moveable pulley. Be sure
that the scale is zeroed.
2.
Fasten the 100 g mass (1 N) directly to the pulley.
3.
Pull the spring scale in an upward vertical direction. In Data Table 3, record the
reading in the effort force (Fe) column to the nearest .5 N.
Figure 2
4.
Measure the vertical distance (in meters) your effort moves when the resistance force moves
a vertical distance of .10 m. Record the effort distance (de) in Data Table 3.
5.
Repeat steps 2 and 3 using the resistance forces and resistance distances listed for Trials 2
and 3 in Data Table 3.
6.
Using the Win, Wout , and MA formulae, calculate the values for work input, work output and
ideal mechanical advantage for each of the three trials. Record the answers in Data Table 4.
DATA TABLE 3: Single Moveable Pulley
Resistance
Effort ForceEffort Distance
Distance
(Fe)
(de)
(dr)
DATA TABLE 4: Single
Moveable Pulley Work & MA
Work
Work output
Trial
Input
MA
(Wout)
(Win)
Trial
Resistance
Force
(Fr)
1
1N
.10 m
_____N
_____m
1
_______Nm
_______Nm
2
2N
.15 m
_____N
_____m
2
_______Nm
_______Nm
3
3N
.20 m
_____N
_____m
3
_______Nm
_______Nm
QUESTIONS: Part 2-Single Moveable Pulley
1. How did the effort force compare to the resistance force in each trial?
a. Fe < Fr
b. Fe > Fr
c. Fe = Fr
2. How did work input compare to work output for the 3 trials?
a. Win < Wout
b. Win > Wout
c. Win = Wout
3. Compare the mechanical advantage in the 3 trials.
a. MA increased
b. MA decreased
c. MA remained equal
4. What kind of help or advantage does a single moveable pulley provide? (choose one)
a. Multiplies effort force
b. multiplies distance/speed c. changes direction
5. Name 1 place you might find a single moveable pulley (other than science class).
Multiple Pulley Systems
Pulleys can be arranged to give a greater mechanical advantage than those you have been using
so that heavier weights can be lifted. In order to do so, several pulleys must be used together and
connected in certain ways. In this part of the investigation, you will be using combinations of single
fixed, single movable, double fixed and double movable pulleys. You will be connecting them in
different ways and finding the mechanical advantage of each combination.
49
PROCEDURE: Part 3 – Multiple Pulley
Systems
1. Arrange the pulleys as shown in systems
A, B, C, and D and record the data
collected in Data Table 5.
2. For each system, measure the effort
force needed to lift a 5 N (500 g)
resistance force.
A
B
C
D
3. Measure how far the effort force of each system must move to raise the weight a distance of
.10 m.
4. Calculate the ideal mechanical advantage of each system and record in Data Table 5.
5. Count the number of strings that support the weight in each of the four systems.
DATA TABLE 5: Pulley Systems
System
Resistance
Force
(Fr)
Resistance
Distance
(dr)
Effort
Force
(Fe)
Effort
Distance
(de)
A
5N
.10 m
_____N
_____m
B
5N
.10 m
_____N
_____m
C
5N
.10 m
_____N
_____m
D
5N
.10 m
_____N
_____m
# of support
strings
MA
QUESTIONS: Part 3 – Pulley Systems
1. The MA of system A is (<, >, =) the MA of system B.
2. The MA of system B is (<, >, =) the MA of system C.
3. The MA of system C is (<, >, =) the MA of system D.
4. What happened to the MA as the difference between de and dr increased?
a. MA increases b. MA decreases
c. MA stays the same
5. What is the relationship between mechanical advantage and
the number of supporting strings?
6. Study the pulley systems X, Y, & Z. Predict the MA of each
of the pulley systems by using the number of support strings.
a. MA of system X = ____________
b. MA of system Y = ____________
c. MA of system Z = ____________
7. Draw a pulley system with a MA = 6 that pulls down.
ANALYSIS & REFLECTION:
X
50
Y
Z