## Pulley Power

### Richard Angelini Sr.

#### Description

This lesson is a manipulative experiment in the mechanical advantage of simple machines and graphically demonstrates the change in magnitude of applied force when using simple machines. Great for ESL to discover the ratio formula for pulleys.

#### Objectives

The student knows that simple machines can be used to change the direction or size of a force.

#### Materials

-1 spring scale
-1 single pulley
-1 double pulley
-30 feet of cord
-1 8 lb. weight (use a gallon jug of water)
-1 2” X 2” piece of wood 8 feet long
-Some duck tape or string, or wire, or something to attach pulleys

#### Preparations

1. Use a marker to mark the 2X2 in 10cm intervals.
2. Attach the top pulley with duck tape and thread cord through both pulleys.
3. Fill gallon jug with water.

#### Procedures

Introduction:
Ask the Students: “Who can lift 1,000 pounds?- Explain to the students that the answer is 'everyone'. Explain that all that is needed is a simple machine. Now, explain how it works.
- A pulley extends the distance you have to lift, but in a good way. Let’s do this simple experiment. Take notes. Your notes will be assessed.” At this time share the criteria listed in the assessment section with students. Do not share, however, the conclusion that they must make.

Procedure:
1. Have two students help. One student holds the 2X2 steady and vertical. The other student will attach the weight to the bottom of the pulley in this fashion:
a. Attach a single wheel pulley to the top of the 2X2.
b. Thread the cord or rope through the pulley and tie one end to the weight.
c. Attach to the other end the spring scale.
d. Make sure that there is at least 4 feet of cord or rope on each side of the pulley.

2. Now direct the students to pull on the rope. Tell them to observe the spring scale and record the weight indicated. Tell the students: This use of a pulley is called a hoist. It is used to change the direction of the applied force.

3. Now remove the weight and the spring scale from the pulley rope.

4. Weigh the weight and record. Tell the students: “Notice that the two weights are the same. Record the results. Hoists do not have a mechanical advantage.”

5. Remove the rope and design a three-wheel, double pulley system. Do this by attaching the double wheel pulley to the top of the 2X2 and the single pulley below. Attach the end of the rope to the top of the single pulley and wind the rope through one of the double pulley wheels, back down through the single wheel pulley and then back up thru the second wheel of the double wheel pulley. Next, attach the weight to the bottom of the single wheel pulley. Then, attach the spring scale to the end of the rope you will pull.

6. Tell the students: “ Now observe the scale. When we pull on the spring and therefore the rope, we read a force of 1/3 what it was to move the weight with the single wheel pulley.”

7. The students should have written down in their notebooks an original weight of 8 pounds, (1 gal. water). The single wheel pulley will reverse the direction of the force but the pull will still be 8 pounds. With the 3-wheel pulley system, the force will be 1/3 of 8 pounds, or 2.66 pounds, because the force is spread over three times the distance.

8. Tell the students: “ The mechanical advantage of the 3-wheel pulley over the single wheel pulley is 3:1 because it takes 1/3 of the original force in pounds to move the weight.” This like building an inclined plane to move heavy objects up. Ask the students if they have made this observation.

9. Draw an inclined plane on the board. Show the students that the ramp is longer but it spreads the needed force over a greater distance. This the same thing a pulley system does. To calculate the mechanical advantage if the inclined plane, divide the horizontal distance of the inclined plane by the vertical distance. To calculate the force needed to move the weight on the inclined plane, multiply the weight by the fractional representation of the mechanical advantage. For example, if the mechanical advantage is 4:1, then divide the weight by 4.

10. Tell the Students: “Mechanical advantage changes the magnitude of the applied force.” Tell the students: “The ratio of the force required to lift the weight without the pulley to the amount of force required to lift the weight with the pulley is known as mechanical advantage. Mechanical advantage is used everywhere; in automobile transmissions, electric drills, all kinds of machinery. Now let’s see mechanical advantage.”

#### Assessments

Formative Assessment:
Students should design their own method of recording the experiments. Give points for inclusion of these below listed attributes in their notes.
--2 points if they draw a diagram of both pulley systems.
--3 points if they correctly record the before and after weight of force in both experiments.
--3 points for correctly calculating the mechanical advantage of the 3-wheel pulley system.
--1 point for drawing an inclined plane.
--3 points for the correct statement: “Mechanical advantage changes the magnitude of the applied force,” recorded in their notes.
Total: 12 points

Students who have problems with the criteria will need individual corrective feedback about what is expected.
Students also take the Summative Assessment, Pre and Post Test, found in the associated file when the teacher feels the students are ready. An answer key is provided.