After this activity, students should be able to:

• Describe how pulleys make work easier for engineers.
• Demonstrate the difference between fixed and movable pulleys.
• Demonstrate how using or combining multiple simple machines can further affect work.
• Describe everyday examples of pulleys used by engineers.

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Imagine that the whole class is on the roof of a tall building, looking down at the street below. On the street are the parts for a roller coaster that we want to install on the top of the building. Our problem is that the pieces do not fit into the elevator and the building is much too high for us to simply lift it up to the roof. How can we possibly get these roller coaster parts, which weigh about 1,000 kilograms each, to the roof in one piece? Are there any simple machines that would help? We could use inclined planes and go back and forth up the side of the building but that would be dangerous and it would take too much time.

The best answer is to use pulleys! A pulley is a wheel and axle with a rope over it. A pulley is one type of simple machine. Pulleys enable us to get big items up to the roof without any trouble at all. But, what route do we take using the pulleys? We have so many options…

• What if we tie a rope to the roller coaster part and then put the rope over a pulley placed on the edge of the roof? The free end of the rope can be pulled by all the students on the roof (like a tug of war) to bring the part up to the roof. It would take a lot of effort, but we could do it (assuming we do not fall off the other side of the building first!).
• Hmmm… Falling off the other side of the building might not be so bad after all… or at least having the rope fall off the other side of the building. If we used another pulley on the other side of the building and a longer rope we could pull down on the rope from the ground on the other side of the building. Pulling down would be easier than pulling up.
• Both of those options require that we use 1,000 kilograms of force to raise the part. If we attach a 500-kilogram weight to our side of the rope and then pull, we would only have to pull with 500 kilograms of force. In this case, we would be using gravity to our advantage and making our work easier.

All of these options are examples of fixed pulleys. There is also a movable pulley option. Attaching the pulley to the roller coaster part, one side of the rope to the top of the building and us pulling on the other side of the rope would enable us to do the same thing as the first two options. We could also create a mechanical advantage by using a fixed pulley and a movable pulley together, as a pulley system. It would take twice as much rope, but only half as much strength.

Engineers think of ways to do tasks — such as lifting heavy objects or even lifting light objects high above our heads — everyday. We see examples all around us, for example, a crane that uses several pulleys lifts heavy objects (see Figure 1) on a construction site. Something as simple as the flagpole outside the school uses a pulley to raise a light object, the flag, high above our heads. Do you see how valuable it can be to use a pulley to redirect the direction of a force?

Pulleys are not always used for lifting objects. Sometimes the purpose of a pulley is to move object or provide a continuous line. A bicycle chain uses two pulleys and a chain that continues to go around in circles.

Engineers make complex machines by combining together several simple machines, so that many difficult tasks may be done more easily. For example, using a large screw at one end of the rope and turning the screw like a crank would enable even easier rising of the roller coaster parts.

Today, we are going to look more at fixed pulleys, movable pulleys and pulley systems. Then, we can use what we learn to think about how we could use pulleys to engineer an ancient pyramid.

(optional) To set the stage for this activity, show students the attached Wheeling It In! PowerPoint presentation. Show the PowerPoint presentation, or print out the slides to use with an overhead projector. The presentation is animated to promote an inquiry-based style; each click reveals a new point about each machine; have students suggest characteristics and examples before you reveal them.

(optional) Print out the attached Pulley Visual Aids (on paper or overhead transparencies) to review fixed and movable pulleys, and pulley systems with the students.

Figure 2. Using a block and tackle reduces the amount of force needed to raise an object. Many boats have devices like this to enable people to raise extremely heavy objects by hand. click for copyright

Before the Activity

With the Students

1. Divide the class into teams of two students each.
2. Distribute all materials to the teams (2 pulleys, string, worksheet and 1 object to lift for each group). Make available tape and a spring scale, either for each team or to share among the teams.

Figure 3. Activity supplies to make a pulley. click for copyright

3. Tie one end of the string to the object to lift.
4. Tie a small loop in the other end of the string.

   Figure 4. Fixed pulley. click for copyright

5. Weigh the object by hooking the loop of string to the hook on the spring scale and pulling the object straight up in the air. Record this value on the worksheet.
6. Create a fixed pulley by running the string over the pulley wheel.
7. If using spools, have students create a pulley by placing a pencil, chopstick or straw through the middle of the spool as an axle (see Figure 3). Make sure the spool is able to freely rotate around the axle.
8. Have one student hold the pulley still and another pull the string sideways or down, as shown in Figure 4. Record the force needed (measurement on the spring scale) on the worksheet.

9. Next, create a movable pulley by attaching the pulley to the object (see Figure 5). If using pre-fabricated pulleys, attach with tape. If using spools, attach the pulley axle to the object with tape or string, making sure the spool still rotates freely.)
10. One student holds the string in place while another student lifts the object by pulling up on the spring scale string, as shown in Figure 5.

Figure 5. Movable pulley. click for copyright

11. Observe how much force is required to lift the object (spring scale measurement) and record it on your worksheet.
12. Last, create a pulley system. Run the string through one pulley (the fixed pulley) and through the other (the movable pulley, which has the object attached to it), as in Figure 6.
13. Have one student hold the free end of the rope and the fixed pulley, while another student raises the object by pulling down on the spring scale. Record on the worksheet the force required to lift the object (spring scale measurement) with this two-pulley system.

Figure 6. Pulley system. click for copyright

14. Provide time for the student teams to complete the remainder of the worksheet.
15. Conclude by comparing results as a class, and discussing the mechanical advantages seen by using the various pulley configurations.

Depending on resources, stations could be established with different pulley systems for the students to rotate through. Or, set up example pulley systems and have students re-create their own systems.

If straws bend due to the weight of the object (some objects may be too heavy for straws), use chopsticks, pencils or toothpicks.

The spool should spin freely on its axle. If the pulley does not rotate freely, the axle may be too large; try a thinner item for an axle, such as toothpicks or wooden cooking skewers.

If not enough pulleys are available, have two teams pair up to make the pulley system (steps 12 and 13.)

If the force recorded for the two-pulley system measures larger than the other forces recorded, it is likely because there is too much friction in the spools. Re-arrange the spools so they move freely around the axle and try again. This could also lead to a class discussion on friction and how engineers must design pulleys to have as little friction as possible. Use pre-fabricated metal or plastic pulleys to demonstrate the reduced friction compared to thread spool pulleys.

Figure 7. This class demonstration helps students to feel the mechanical advantage of using multiple pulleys. Running a rope over the rod many times greatly reduces the amount of force required to lift the jug. click for copyright

The class may benefit from a demonstration using heavier objects than those used in the activity, so that they really notice that the amount of force required drops when using multiple pulleys. Use a plastic milk or juice jug filled with sand or liquid. Tie a rope to a strong bar (or rod) above your head (a swing set works well). Run the rope through the handle of the jug and then over the bar overhead. Raise the jug by pulling the rope down; let students feel how difficult it is to do this. Next, run the rope through the jug and over the bar again. Raise the jug again. Continue to loop the rope through the jug and over the bar noting how much easier it becomes to raise the jug (see Figure 7); this is a good example of how a block and tackle (see Figure 2) works. (Source: http://www.grc.nasa.gov/WWW/K-12/Summer_Training/KaeAvenueES/pulleys.html)

Dictionary.com. Lexico Publishing Group, LLC. Accessed February 1, 2006. (Source of some vocabulary definitions, with some adaptation) http://www.dictionary.com

Handtools for Trail Work, Tools for Lifting and Hauling, Block and Tackles. Updated February 1, 2006. Environment, Federal Highway Administration, U.S. Department of Transportation. Accessed February 1, 2006. (Source of block and tackle image.) http://www.fhwa.dot.gov/environment/fspubs/05232810/page16.htm

Huddle, Carol. Pulleys. Simple Machines Lesson Plans, Glenn Learning Technologies Project, Glenn Research Center, NASA. Accessed February 1, 2006. (Source of Activity Extension.) http://www.grc.nasa.gov/WWW/K-12/Summer_Training/KaeAvenueES/pulleys.html

Tensile Strength Test Strand. Space Team Online, NASA Quest. Accessed February 1, 2006. http://quest.arc.nasa.gov/space/teachers/suited/9c1ten.html