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TE Activity: Power, Work and the Waterwheel

Waterwheels are devices that generate power and do work. Students construct a waterwheel using two-liter bottles, dowel rods and index cards, and calculate the power created and work done by them.

Civil, geotechnical, environmental mechanical and electrical engineers collaborate to design and construct dams that generate electricity from the flow of water. When engineers design these dams, called hydroelectric power plants, they calculate the amount of power that can be generated by the plant. Knowing the dam's potential power generation, they can further estimate the maximum rural or urban region that can be supplied with electricity generated from the dam.

Learning Objectives
Materials
Introduction/Motivation
Procedure
Attachments
Safety Issues
Troubleshooting Tips
Assessment
Extensions
Activity Scaling
References

Grade Level: 7 (6-8)	Group Size: 4
Time Required: 45 minutes	Activity Dependency : None
Expendable Cost Per Group : US$ 1.50
Keywords: energy, waterwheel, work, power, potential energy, kinetic energy, Newtons, Joules, Watts

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Colorado Math
- 1. represent, describe, and analyze patterns and relationships using tables, graphs, verbal rules, and standard algebraic notation; (Grades 5 - 8) [2005]
- 1. demonstrate meanings for integers, rational numbers, percents, exponents, square roots, and pi (π) use physical materials and technology in problem-solving situations; (Grades 5 - 8) [2005]
- 1. estimate, use, and describe measures of distance, perimeter, area, volume, capacity, weight, mass, and angle comparison; (Grades 5 - 8) [2005]
- 2. construct, use, and explain procedures to compute and estimate with whole numbers, fractions, decimals, and integers; (Grades 5 - 8) [2005]
Colorado Science
- Standard 1: Students understand the processes of scientific investigation and design, conduct, communicate about, and evaluate such investigations. (Grades 0 - 12) [1995]
- Standard 5: Students know and understand interrelationships among science, technology, and human activity and how they can affect the world. (Grades 0 - 12) [1995]
- 2.3 Students understand that interactions can produce changes in a system, although the total quantities of matter and energy remain unchanged. (Grades 0 - 12) [1995]

Power and work are important concepts that impact the engineering design of items ranging from racecar engines to elevators to power plants. High-power cars (high-horsepower) are able to accelerate very quickly and go very fast. Elevators in skyscrapers require enough power to lift many people quickly, to avoid long elevator waiting lines. As we'll see, power plays an integral role in the production of hydroelectricity.

Work is measured in Joules (J) and is defined as a force acting over a distance or:

Work = force x distance

In our activity today, work will be done lifting a weight. The force term equals the weight and the distance term equals the height lifted. Power is measured in Watts (W) and is defined by how fast work is done or:

Power = Work ÷ time

In this activity, you are working for H₂O Solutions, an engineering design firm that works mostly with waterwheels and water energy! Your city wants to use hydropower instead of coal to make energy because they are worried about air pollution. The city has hired you to design an efficient watermill. The firm (our class) has been split into several engineering teams (student groups). Each engineering team will design and test a slightly different design so that the firm can present the most efficient design to the city. You will calculate power and work by measuring force, distance and time for your team-built waterwheel.

Before the Activity

Gather materials and make copies of the H₂0 Solutions Worksheet, one per person.
Drill 3/8-inch holes into the end of the two-liter bottle and the cap. This allows the bottles to spin symmetrically and freely about the dowel rod. (If you don't have the hole in the cap, the dowel rod will not spin symmetrically.)

With the Students

A drawing illustrates the activity setup, showing water poured through a funnel onto a two-liter bottle. The water hitting the index card fins causes the bottle to spins about a horizontal dowel-rod axis. As the bottle spins, a string tied near the cap end of the bottle wraps around the bottle neck and pulls up a weight.

Figure 1: Illustration of the activity setup.
click for copyright

Divide the class into groups of four and pass out the materials.
Remind students of the context of the design challenge (they are engineers working for a firm hired to design an efficient water wheel). They should keep track of their design process using the worksheet.
Instruct the students to attach the index cards to the sides of the two-liter bottle to create a waterwheel (open-ended).

Encourage the students to brainstorm different ideas of where to place the index cards.
Explain that the water will be supplied from a pitcher through a funnel and the bottle will spin on the dowel rod.

When the students are finished with their design, have them tie the string to the cap end of the bottle so that when the bottle rotates, the string wraps around the bottle neck, pulling up the string.
Measure and tie a weight to the other end of the string.

Make sure to record the mass of the object in kilograms (kg). (For example, 100 grams is 0.1 kg.) Multiply the mass by gravity (~10) to calculate your force.
Have everyone use about the same amount of weight.
Make sure the weights are not too heavy to lift. (For example, 100-200 gram weights work well)

Test the waterwheels by pouring water through a funnel to achieve an even flow, and timing how long it takes to lift the weight 1 meter (This is your distance).

Perform this test outside or over a sink.
Have two students hold the ends of the dowel rod, one student pour the water and one student time how long it takes and write it down.
Make sure the funnel is only a couple of inches above the waterwheel each time.

Have the students calculate the work and power of their waterwheel.

Work = force x distance
Power = Work ÷ time

Which team had the most power? (Answer: They will all do the same amount of work, but faster wheels will have more power.)

Pre-Activity Assessment

Brainstorming: In small groups, have the students engage in open discussion. Remind students that no idea or suggestion is "silly." All ideas should be respectfully heard. Write down all the groups' ideas on the board to share with the class.

"What features make a good waterwheel?" (Possible answers: i.e., a lot of fins/index cards to turn the wheel/bottle, each fin/index card holds a large amount of water, symmetry, etc.)

Activity Embedded Assessment

Prediction: Have each student group predict how their waterwheel is going to do and why. Based on their prediction, ask each group if their wheel will do more work or have more power than the other groups. (Answer: They will all do the same amount of work, but faster wheels will have more power.)

Post-Activity Assessment

Question/Answer: Have students answer the following question in a short paragraph.

Explain the difference between work and power in your own words. (Answer: Work is a force acting over a distance and is measured in Joules and not dependent on time. Power is work divided by time and is measured in Watts.)

Question/Answer: Put the following problem on the board (or overhead projector) and have the students solve it:

Mr. Muscles loads up a bar with 910 Newtons (≈205 lbs) of weight and pushes the bar up over his head 8 times. Each time he lifts the weight .5 meters. How much work did he do? If he does the whole thing in 15 seconds, how much power did it take? (Answer: Work = 3640 Joules. Power = 242.7 Watts. See work, below.)

Work = Force x Distance

Force = 910 Newtons

Distance = .5 meters x 8 = 4 meters

Work = 910 Newtons x 4 meters = 3640 Newton•meters = 3640 Joules

Power = Work ÷ time = 3640 Joules ÷ 15 sec = 242.7 Joules/sec = 242.7 Watts

Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder Bailey Jones, Matt Lundberg, Chris Yakacki, Malinda Schaefer Zarske, Denise Carlson © 2004 by Regents of the University of Colorado.
The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0338326. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.

Last Modified: October 3, 2007