- University of Virginia
- Physics Department
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Pulleys
A Physical Science Activity
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2003 Virginia SOLs
Objectives
Students will
- construct pulley systems using one or two pulleys;
- understand the relationship between the arrangement of the pulleys and the
effort required to lift a load;
- learn various applications of pulleys in everyday life.
Motivation for
Learning
Discrepant Event
- Materials
- 2 broomsticks
- 1 meter of thin rope
Procedure
- Ask for two volunteers from the class to help.
- Tie one end of the rope to a broomstick. Wrap the other end
once around the second broomstick.
- While the two volunteers are trying to hold the broomsticks
apart, attempt to pull them together using the free end of the
rope.
- To make it easier, wrap the free end of the rope around the
first stick another time and pull.
- Again, to make it easier, wrap the rope around the second
stick another time and try to pull them together.
- This system works similarly to a pulley--therefore, as the
number of wraps increases, so does the mechanical advantage
provided to you. Ask the class to explain this analogy.
Background
Information
The pulley is a simple machine that consists of a grooved wheel
and a rope. Like a lever, it provides a mechanical advantage in
lifting a heavy load. There is a direct relationship between the
number of ropes that form the pulley and its resulting advantage.
There are two basic types of pulleys. When the grooved wheel is
attached to a surface it forms a fixed pulley. The main benefit of a
fixed pulley is that it changes the direction of the required force.
For example, to lift an object from the ground, the effort would be
applied downward instead of pulling up on the object. However, a
fixed pulley provides no concrete mechanical advantage. The same
amount of force is still required, but just may be applied in another
direction. Another type of pulley, called a movable pulley, consists
of a rope attached to some surface. The wheel directly supports the
load, and the effort comes from the same direction as the rope
attachment. A movable pulley reduces the effort required to lift a
load.
These two types of pulleys can be combined to form double pulleys,
which have at least two wheels. There are various combinations which
can result in a double pulley, some of which will be explored in the
student experiment. As the pulley becomes more complex, the total
lifting effort decreases. For example, a system consisting of a fixed
pulley and a movable pulley would reduce the workload by a factor of
two, because the two pulleys combine to lift the load.
In this experiment, the students will use a spring scale to record
the required effort. The spring scale itself has a small amount of
mass and so will contribute a small force to the effort because of
gravity. To determine this force, the students should hang one spring
scale from another and note the reading from the top scale
(approximately 0.5 N). This amount should be added to the forces
recorded throughout the procedure when the spring scale is being
pulled towards the ground (in a fixed pulley). Remember to explain
this to the students before they begin the activity.
Student
Activity
To print out the Student Copy only,
click
here.
Materials
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- Gravel (240 g) per bottle
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- 2 Plastic bottles with screw eye in lid, 500 ml (can
be ordered from Delta Education Foss Catalog
(1-800-258-1302); item number 420211232)
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Procedure
- Hang one spring scale from another. Read
the measurement from the top scale (the top of the curved line)
and record it on the data table.
- Fill each plastic bottle with gravel.
Check the mass on a scale; if it is not 240 g, add water until the
mass is corrected.
- Cut the cardboard into two pieces. One
should be 30x50 cm, the other 20x50 cm.
- On the smaller piece of cardboard, trace
the size of the lid of a plastic bottle. Cut a square around the
shape, and then cut the shape of the lid from the cardboard. This
square should be placed on the bottle before the lid is screwed
on.
- Tape two pieces of white paper on the
larger piece of cardboard, one above the other. On the lower
sheet, use a ruler to mark dashes at 5 cm, 10 cm and 15 cm from
the bottom of the cardboard. Draw lines across the paper at these
dashes.
- Clip the binder clip to the end of the
ruler, so that the metal rings face towards you.
- Tape the ruler to a desk or table so
that the clip hangs over the edge of the surface. Set a book on
the ruler to ensure that it doesn't slide off.
- Fold 5 cm from the end of the rope over
into a loop and secure the loop with masking tape. Repeat on the
other end of the rope.
- Construct a fixed pulley. First, attach
the pulley wheel to the binder clip on the edge of the surface.
Thread the rope through the pulley. From one end of the rope, hang
the load. From the other, hang the spring scale.
- Lift the load and record the required
effort. Add this to the weight measured in step 1 of the spring
scale.
- Now construct a movable pulley. To do
this, attach one end of the rope to the binder clip. Thread the
rope through the pulley wheel, and attach its other end to the
spring scale. Hang the load from the wheel itself. For a movable
pulley, the load should be hanging from the center of the rope,
and the spring scale should be above it.
- Lift the load and record the required
effort. Add to this the weight of the spring scale measured in
step 1.
- Retrieve the large sheet of cardboard
with the white paper on it. Tape this sheet to the side of the
table or counter, directly underneath the binder clip.
- Set up a fixed pulley system as you did
in step 5.
- Hang the load from one end of the rope.
Put a pencil through the loop at the other end (instead of the
spring scale).
- Apply effort to the pencil so that the
load hangs at the lowest (5cm) line drawn on the white paper. Use
the cardboard collar on the load to judge this.
- Keep the pencil in the rope loop for the
rest of the experiment. While the load is positioned at the 5cm
line, draw a line on the white paper with the pencil where it
hangs.
- Drag the pencil down the paper, drawing
a line while lifting the load so that the collar reaches the top
line (15cm).
- Release the load. Measure the distance
traveled by the effort (the pencil line on the paper). Record this
on the data table.
- Set up a movable pulley system as in
step 8. Hang the load from the pulley. Again, replace the spring
scale with a pencil.
- Repeat steps 12-14. With this setup,
however, the pencil will move up along the paper.
- Record the distance traveled on the data
table.
Data Sheet
To print out the Data Sheet only,
click here.
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Force of spring scale itself:
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Direction of Effort
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Load (N)
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Effort (N) + spring scale force
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Effort Distance
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Load Distance
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Fixed Pulley
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Movable Pulley
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Questions
1. Is there a difference in the effort required to lift a
load when using a fixed pulley and a movable pulley?
Explain.
2. Did the effort distance differ when using the two
types of pulleys? Why or why not?
3. Describe a situation in which it would be to your
advantage to use a pulley instead of another type of simple
machine.
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Students with Special Needs
Some students may have difficuilty maniupulating the objects necessary for
the assembly of the apparatus. This activity can be done with partners or in
small groups.
Click here for further
information on laboratories with students with special needs.
Assessment
Data sheet to be completed during the laboratory.