University of Virginia
Physics Department

Pulleys

A Physical Science Activity

2003 Virginia SOLs

 

Objectives

Students will

 

Motivation for Learning

Discrepant Event

Materials
 

Procedure

  1. Ask for two volunteers from the class to help.
  2. Tie one end of the rope to a broomstick. Wrap the other end once around the second broomstick.
  3. While the two volunteers are trying to hold the broomsticks apart, attempt to pull them together using the free end of the rope.
  4. To make it easier, wrap the free end of the rope around the first stick another time and pull.
  5. Again, to make it easier, wrap the rope around the second stick another time and try to pull them together.
  6. 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

  • ½ meter stick or ruler
  • Rope, 70 cm long
  • Binder clip, medium size
  • 2 Paper clips
  • Spring scale, 10 N max.
  • Cardboard, 50x50 cm
  • Gravel (240 g) per bottle
  • White paper
  • Rubber band
  • Masking tape
  • 2 Single pulleys
  • 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)

Procedure

  1. 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.
  2. 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.
  3. Cut the cardboard into two pieces. One should be 30x50 cm, the other 20x50 cm.
  4. 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.

  5. 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.

     

  6. Clip the binder clip to the end of the ruler, so that the metal rings face towards you.
  7. 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.
  8. 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.
  9. 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.

     

  10. Lift the load and record the required effort. Add this to the weight measured in step 1 of the spring scale.
  11. 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.

     

  12. Lift the load and record the required effort. Add to this the weight of the spring scale measured in step 1.
  13. 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.
  14. Set up a fixed pulley system as you did in step 5.
  15. Hang the load from one end of the rope. Put a pencil through the loop at the other end (instead of the spring scale).
  16. 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.
  17. 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.
  18. Drag the pencil down the paper, drawing a line while lifting the load so that the collar reaches the top line (15cm).
  19. Release the load. Measure the distance traveled by the effort (the pencil line on the paper). Record this on the data table.
  20. Set up a movable pulley system as in step 8. Hang the load from the pulley. Again, replace the spring scale with a pencil.
  21. Repeat steps 12-14. With this setup, however, the pencil will move up along the paper.
  22. Record the distance traveled on the data table.

 

Data Sheet

To print out the Data Sheet only, click here.

Force of spring scale itself:

Direction of Effort

Load (N)

Effort (N) + spring scale force

Effort Distance

Load Distance

Fixed Pulley

 

 

 

 

 

Movable Pulley

 

 

 

 

 

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.

 

 

 


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.