18.1: Comparing Baths and Showers (5 minutes)
This warm-up sets the stage for this lesson. Students are presented with the basic question of whether baths or showers use more water and they brainstorm information that might help them investigate the question.
"Some people take showers, some people take baths. There is disagreement over which one takes more water. What do you think?" Ask students to think for just a minute about whether they think a shower or a bath uses more water. (This is just to record their first instinct—they should not spend any time researching or calculating right now.) Poll the class and record the total for each category for all to see: "think a bath takes more water" and "think a shower takes more water."
Some people say that it uses more water to take a bath than a shower. Others disagree.
- What information would you collect in order to answer the question?
- Estimate some reasonable values for the things you suggest.
Invite students to share their responses. Record and display their responses for all to see. If these quantities do not come up in conversation, ask students to discuss the ideas and provide reasonable estimates:
- Time spent in the shower
- Volume of the bath tub
- Rate of water coming out of the shower head
18.2: Saving Water: Bath or Shower? (20 minutes)
When students are finding values to aid in their method, consider allowing them to research typical values online at hardware websites or search for values that would be useful. If these tools are not available, some values are provided here.
Values that may be useful for students:
- Typical (modern) shower heads have a flow rate of 1.9 to 2.5 gallons per minute. Older shower heads (pre-1992) could have flow rates up to 5.5 gallons per minute.
- Bath tubs hold approximately 120 to 180 gallons of water when completely filled to the top.
- The interior of a typical bath tub has an approximate width of 30 to 32 inches, length of 55 to 60 inches, and depth of 18 to 24 inches.
- There are approximately 230 cubic inches in 1 gallon of water.
- 1 liter of water is 1,000 cubic centimeters.
- 1 liter is approximately 0.26 gallons
- 1 inch is 2.54 centimeters.
- Typical showers last approximately 11 minutes although during a drought, it is recommended to reduce the time to about 5 minutes. During normal circumstances, some people appreciate much longer showers.
After students have made good progress in the activity, tell them to make a display (e.g. poster) to share their method and results.
This activity can take the rest of the class period, if desired.
Arrange students in groups of 2–4. Tell them either that they should research relative information and provide access to internet enables devices or tell them that they can ask for information they need.
Supports accessibility for: Memory; Organization
- Describe a method for comparing the water usage for a bath and a shower.
- Find out values for the measurements needed to use the method you described. You may ask your teacher or research them yourself.
- Under what conditions does a bath use more water? Under what conditions does a shower use more water?
Allow students to share their displays possibly through a gallery walk or ask them to present to the class. After students have had a chance to explore their work, ask them to share ideas they saw that were interesting and any methods they considered but did not use.
Design Principle(s): Maximize meta-awareness; Support sense-making
18.3: Representing Water Usage (10 minutes)
This optional activity gives additional review for the material from the unit in the context of this lesson. Students build on the work they did in the previous activity and see that water used in the shower is proportional to time spent in the shower, with constant of proportionality equal to the flow rate of the shower head.
Keep students in the same groups.
Fine Motor Skills: Peer Tutors. Pair students with their previously identified peer tutors and allow students who struggle with fine motor skills to dictate graphing as needed.
Supports accessibility for: Organization; Conceptual processing; Attention
Design Principle(s): Optimize output (for explanation); Maximize meta-awareness
- Continue considering the problem from the previous activity. Name two quantities that are in a proportional relationship. Explain how you know they are in a proportional relationship.
- What are two constants of proportionality for the proportional relationship? What do they tell us about the situation?
- On graph paper, create a graph that shows how the two quantities are related. Make sure to label the axes.
- Write two equations that relate the quantities in your graph. Make sure to record what each variable represents.
Instruct students to add this information to their displays from the previous activity. Invite students to share their reasoning.
Consider asking discussion questions like these:
- “Do you agree or disagree? Why?”
- “Did anyone solve the problem the same way but would explain it differently?”
- “Who can restate ___’s reasoning in a different way?”
- “Does anyone want to add on to _____’s strategy?”