OBJECTIVES: You will learn about simple pulley systems and how to tell the difference between a simple system and a complex pulley system. You will build a simple pulley system using pulleys and weights to demonstrate the mechanical advantages of pulleys. STEPS TO FOLLOW: Discuss the information in the background section with the students. Be sure to emphasize the difference between moveable and fixed pulleys, simple and complex pulley systems, and the concept of equilibrium. Collect the materials for the demonstrations. Have the cup hook screwed into the wooden slat. Have small loops of twine tied around the weight(s). Cut a length of twine and put a slip knot in one end. This will make it easier to trade off from one pulley system to another. Place the slat across 2 chairs, hook down, with the books on either side to hold the slat secure. Attach one pulley at the hook. Attach the length of twine to the weight with the slip knot and thread the twine through the pulley so that the students can pull down on the twine. Discuss the mechanical advantage (1) and how much force it takes to hold the weight up and steady.

MATERIALS: 2 or more small pulleys (can be purchased at a hardware store) Weights (2 1/2 lb, 5 lb, for example) Slat of wood Cup hook Medium weight twine Scissors S-hook 2 chairs 2 or more heavy books Attach the slip knot to the hook, and use the S-hook to attach the weight to the pulley. Thread the twine around the pulley and let the students pull up to hold the weight steady. Ask them if it feels lighter (less effort to pull up than before). Have them compute the mechanical advantage (2). How much force are they using to hold it up? (W/2) Using the slip knot, tie the twine to the center of one pulley and hang it from the hook. Thread the twine around the bottom pulley with the weight attached and back through the top pulley. The students should pull down on the twine and feel the same weight as in step 4. Have them show that the mechanical advantage is still 2. Now tie the twine to the bottom pulley and thread it around the top pulley and down through the bottom pulley. Hang the weight off the bottom pulley. Have the students pull up on the twine. It should take even less effort to hold the weight up. See if they can compute the mechanical advantage of this system (3). Variation: If an accurate fishing scale is available (test it on the purchased weight to check for its accuracy), the input force (the pull) could be measured. It should be fairly close to the calculated effort when the system is in equilibrium.