Pulleys

INTRODUCTION

The goal of this experiment is to demonstrate how pulleys lower the amount of applied force necessary to move a load.

MATERIALS

• 1 7/8" diameter dowel rod
• 3 pulleys
• 1 500 gram weight
• 1 spring balance or a pail or cup with gram weights
• Rope

WHAT TO DO

1. First use a spring balance to measure the amount of force needed to raise the 500 g weight to a specified height.
2. Next attach a clamp pulley to the dowel rod and attach the weight to the pulley, measure how much force is needed raise the 500 g weight to the same height as in part 1. Try to measure the length of rope that you pulled to get the weight to move to the desired height.
3. Add a second pulley to the system, and again measure the amount of force needed to raise the weight to the same height as in the two previous measurements. Again try to measure the length of rope that you pulled to get the weight to move to the desired height.
4. Add a third pulley to the system and again measure the amount of force needed to raise the weight to the same height as in the previous sections.

QUESTIONS

1. Compare the information collected in parts 1, 2, 3 and 4.
2. Does it make any difference whether you use one, two or three pulleys to raise the load?
3. Compare the length of rope needed to raise the load in each case. What trend do you observe in your data?
4. Calculate the mechanical advantage of using the two and three pulley systems.

SUMMARY

The use of one pulley only changes the direction of the applied force. When two pulleys are used, the force necessary to raise the weight should be approximately half that needed when using the single pulley. However, friction causes the addition of extra effort. Pulleys trade force necessary to do work for the distance traveled. The distance that the rope had to be pulled with the second pulley should have been approximately twice that used with one.
Add a third pulley to the system. Now there are three sections of rope that have to move up before the load moves. For this case, the force (or effort) necessary to raise the load should be 1/3 that which is needed with one pulley.

SOURCES

"Science Factory: Work and Simple Machines." Jon Richards, Cooper Beech Books, Brookfield, 2000. ISBN: 0-7613-1159-9
"Pulley Activities." NES Arnold, World Class Learning Materials, Baltimore, ISBN: 1-884461-08-5.

Grade Level: This experiment is expected to be appropriate for grades 3 and above.

© S. Olesik, WOW Project, Ohio State University, 2001.