Lesson plan Grade 4 Science
Lifting a Load with the Wexbury Mill Wheel
A 60-minute grade 4 lesson in which groups build a water wheel for a fictional mill, time how fast it lifts three washers, then change one feature and test again.
Objectives
- Students will build a wheel that uses falling water to lift a load of washers.
- Students will change one feature of the wheel and retest it while holding the pour height, the water amount, and the load the same.
- Students will explain where the energy that lifted the washers came from and what it turned into.
Vocabulary
- Hydropower
- Electricity made from moving water, usually water falling through a dam. It is a renewable power source that can start up quickly when the electric grid needs it.
Also: energy, water wheel, axle, blade, fair test, variable
Teacher overview
The fictional town of Wexbury has an old mill on the creek where water falling from a wooden chute once turned a wheel that lifted sacks of grain to the loft, which is the same conversion a hydroelectric dam carries out at a much larger scale. In this lesson groups build a small wheel on a skewer axle, time how long it takes to lift three washers off the table, then change one feature of the wheel and run the same timed lift again at the same pour height with the same amount of water and the same load. The misconception worth planning for is the belief that water has energy only while it is moving, so a full cup resting on the stand is treated as having none until somebody tips it. The baseline trial and the pour-height extension both press on that idea, because the same cup of water does different amounts of work depending on how high it starts.
Procedure
Hook (5 min): Set a full cup of water on a stack of books beside an assembled wheel and ask what the water is able to do while it sits still and what it is able to do while it pours. Record the answers on the board without sorting them, and put a star beside any answer stating that still water has no energy, since the class will come back to that answer at the end.
Building the wheel (15 min): Each group pushes three index-card blades into alternating slits on the cork, slides the cork onto the skewer, and rests the skewer in the notches on the stand. Groups tie one end of the string to the skewer just beside the cork, tape the other end to a paper clip, and hang three washers from the clip. Every wheel spins freely before any water is poured, so check each one and loosen the notches where a skewer binds.
Baseline trial (10 min): Groups mark a pour height on the stand with one strip of tape and a finish line a few centimeters below the axle with a second strip, fill the cup to its line, and pour steadily onto the blades while a partner times how long the washers take to rise from the table to the finish line. Each group runs the timed lift twice and records both times. Groups whose washers do not lift at all should add a second blade set or shorten the string rather than changing the pour height, which has to stay fixed for the comparison ahead.
Planning one change (7 min): Each group picks one change from the posted menu: three blades to six, short blades to long, or flat cards to cards folded into a lip. Groups write down the change and the three things that must stay the same, which are the pour height, the amount of water in the cup, and the three washers. Read two or three plans aloud and ask the class what a group would be unable to say if it changed two things at once.
Refinement trial (13 min): Groups rebuild the wheel with their one change and repeat the timed lift twice from the same pour height to the same finish line, with the cup filled to the same line and the same three washers hanging from the clip. Circulate and stop any group that has altered a second feature, asking that group which of its two changes the new time would be measuring.
Comparison and exit ticket (10 min): Post every group's four times in a class table, grouped by which change was made. Ask which change lifted the load fastest, which made no measurable difference, and how the class can tell. Collect an exit ticket on which each student names the change their group made, one thing the group held the same, and where the energy that lifted the washers came from. If fewer than three quarters of the cards place that energy outside the wheel, in the water held above it, repeat the still-water demonstration before the next science period.
Student Handout
The Wexbury mill wheel. The town of Wexbury has an old mill beside a creek. Water ran down a wooden chute, fell onto the mill wheel, and the turning wheel pulled a rope that lifted heavy sacks up to the loft. Your group will build a small wheel that does the same job with washers instead of sacks. Answer questions 1 and 2 before you pour any water.
Your group will change one feature of its wheel and keep everything else the same. Name the one feature you will change. Then name three things you will keep the same in both trials.
Predict whether your changed wheel will lift the three washers faster, slower, or in about the same time as your first wheel. Give one reason for your prediction.
Both wheels lift the same three washers from the table up to the finish line. Write your first wheel's time and your changed wheel's time in seconds.
Look at your two times. Did your change make the wheel lift the washers faster, slower, or about the same? Use both times as your evidence.
Before you poured, the washers rested on the table, and after you poured they hung in the air. Where did the energy that lifted them come from, and where was that energy before the water left the cup?
Suppose another group raised its pour height and added three blades in the same trial, and its second time came out faster. Explain why that group cannot tell which of the two changes made the wheel faster.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Times and predictions differ from group to group, so the notes below give what to accept rather than one correct number. The mill at Wexbury is fictional, and no group's time is expected to match another group's.
Answer: One named change, with the pour height, the amount of water, and the three washers listed as the things kept the same. Accept blade count, blade length, or blade shape as the change. A group that adds a fourth constant, such as the same person pouring each time, is also correct.
Answer: A prediction of faster, slower, or about the same, with one reason tied to the change. The prediction does not have to be right. Look for a reason about the blades catching more water or catching it better, not a reason about wanting the wheel to win.
Answer: Two times in seconds, one from each wheel.
Check that both timed lifts used the same finish line and the same three washers, because two times taken over different lift distances cannot be compared.Answer: Faster, slower, or about the same, matching the two times the group wrote in question 3. The claim has to agree with the group's own numbers. Times within about a second of each other are better read as no difference than as a win for either wheel.
Answer: From the water, which was held up above the wheel before the pour.
Accept any answer that places the source outside the wheel: the falling water, the water in the raised cup, or the pour. The energy moved from the raised water into the spinning of the wheel and axle, and then into the washers as they rose. An answer saying the wheel made the energy should be returned for a second try.Answer: Two changes at once make it impossible to say which one caused the faster time.
Accept any wording of the fair-test idea, including the suggestion that the group rerun the trial with only the blades changed and the pour height back where it started.
Differentiation
Support
Hand these groups a stand with the pour height already taped and a cork with three blades already fitted, which moves their time from building to testing. Give them a two-column strip headed "what we changed" and "what we kept the same," and narrow the menu to one option, three blades to six. Pair each group with a second group making the same change so the two wheels built for the fictional mill can be compared side by side. The exit ticket may be spoken to the teacher rather than written.
Extension
Ask these groups to run a third trial that changes only the pour height while the wheel itself stays exactly as it was, then explain why that result tells them something about the water rather than about the wheel. A second option is to hang a fourth washer and find out whether the wheel that lifted three washers fastest is still fastest under the heavier load. Groups that finish both can sketch the chute the town of Wexbury would need to feed the mill wheel from the creek.
ELL supports
Pre-teach blade, axle, and lift with the parts in hand, holding up each piece as it is named and turning the axle by hand so the string winds. Provide a labeled diagram of the assembled wheel with arrows running from the cup to the blades to the rising washers. Give sentence frames for questions 4 and 5: "Our second wheel was ___ because ___" and "The energy came from ___." Where a partner shares the student's first language, let the pair retell the pour steps to each other before the timed trial.
Extensions
Students can compare their wheel with a hydroelectric plant, where falling water turns a turbine rather than a mill wheel. Hoover Dam's power plant has a capacity of about 2,080 megawatts, well beyond anything a classroom wheel produces, yet the conversion the class measured is the same one. A second follow-on runs the exchange in the other direction: a utility pump uses electricity to lift water into a tower, while the wheel at the town of Wexbury used water that had already been lifted to do the work. Ask students which of those two conversions is happening at their own water system, and have them find out whether the water reaching their school is pushed uphill by a pump or fed downhill from a tank.