Lab / experiment Grade 8 Health & Consumer Science
Below Glenholt Dam: Cold River Water and Hand Dexterity Lab
Students time a bead task before and after short cool-water hand soaks, plot class data, and explain why cold water below a dam is risky for swimmers.
Objectives
- Students will measure how a short cool-water soak changes hand dexterity using a timed bead-transfer task.
- Students will construct a scatter plot of class data, fit a line informally, and use a linear model to make a prediction.
- Students will use their results to explain why cold water below a dam endangers swimmers and why a life jacket matters.
Vocabulary
Key words: dexterity, tailwater, control, association, outlier, slope
Teacher overview
In our scenario, the Tamber River below the fictional Glenholt Dam stays cold all summer because the dam releases water from deep in its reservoir, where the sun's warmth does not reach. Swimmers who jump in on a hot July afternoon are often surprised by how quickly their hands and arms stop working well. In this lab, students soak one hand in cool water for different lengths of time, count how many beads they can move into a cup before and after the soak, and pool the class results in a scatter plot. Teaching note: many students believe the danger of cold water is hypothermia, which they picture as something that takes hours, so they conclude that a short swim in cold water is safe for a strong swimmer. The more immediate danger is that cold hands and arms lose strength and coordination within minutes, well before the body's core cools, and that is the effect students can see in their own data.
Procedure
Work in pairs. One partner is the tester, and the other is the timer and recorder. Switch roles if time allows.
- Step 1. Get your assignment. Your teacher will give each tester a soak time of 0, 20, 40, or 60 seconds. If cold is painful or unsafe for you because of a health condition such as Raynaud's or cold hives, tell your teacher privately. You will be the timer and recorder instead.
- Step 2. Set up the tubs. Fill one tub with room-temperature tap water. Fill the other tub with tap water and add ice a few cubes at a time. Stir, and check the thermometer. Stop adding ice when the water reads 10 to 12 degrees Celsius. Do not let the cold tub drop below 10 degrees.
- Step 3. Measure the baseline. Put 60 beads in the shallow dish and set an empty cup beside it. The tester uses the hand they write with. On "go," the tester picks up beads one at a time and drops them into the cup for 30 seconds. The recorder counts the beads in the cup and writes the number under "Beads before."
- Step 4. Soak. Return the beads to the dish. A tester assigned 0 seconds puts their writing hand in the room-temperature tub for 60 seconds, so the hand is just as wet as everyone else's. Every other tester puts their writing hand in the cold tub, up to the wrist, for the assigned time. Take the hand out right away if it hurts. Tingling is normal, but pain is a signal to stop.
- Step 5. Measure again. The tester pats the hand dry with a towel for 5 seconds. Then the tester repeats the 30-second bead task at once. Record the count under "Beads after."
- Step 6. Find the drop. Subtract "Beads after" from "Beads before" and record the result. A positive number means the tester moved fewer beads after the soak.
- Step 7. Share your data. Add your soak time and your drop to the class table on the board.
- Safety. Wipe up spills at once, because a wet floor is slippery. Keep beads away from your mouth. Afterward, warm your hand in room-temperature water or tuck it under your arm. Never soak longer than your assigned time.
Data & Analysis
Your result:
| Soak time in cold water (seconds) | Beads before | Beads after | Drop (before minus after) |
|---|---|---|---|
Class results (copy from the board):
| Soak time (seconds) | Drops recorded by testers in this group | Mean drop |
|---|---|---|
| 0 (room-temperature water) | ||
| 20 | ||
| 40 | ||
| 60 |
Make a scatter plot of the class data. Put soak time in cold water on the x-axis and drop in beads on the y-axis. Plot one point for every tester. Describe the association you see, and name any point that looks like an outlier.
Find the mean drop for the 0-second group and the mean drop for the 60-second group. By how many beads is the 60-second mean larger than the 0-second mean?
The 0-second testers soaked their hands in room-temperature water instead of skipping the soak. Why does that make the comparison fairer?
Draw a straight line that fits your scatter plot closely. Are most points close to your line, or are many far from it? What does that tell you about how well a line models this data?
Suppose another class drew a line through the points (0, 1) and (60, 7), where x is soak time in seconds and y is the drop in beads. What is the slope of that line?
Use the line from question 5 to predict the drop in beads for a 45-second soak.
In our scenario, the Tamber River below Glenholt Dam stays cold all summer. A swimmer falls off an inner tube, and a friend on shore throws a rope. Use your results to explain why the swimmer's chance of holding on to the rope gets worse the longer they are in the water.
Your test cooled one hand for one minute at most. A swimmer who falls into the river is cooled all over. Would a life jacket or a plan to swim to shore protect that swimmer better? Explain using your data.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: In most classes, a positive association: longer cold soaks go with larger drops.
Accept any description that matches the student's own plot. Points for the 40-second and 60-second groups usually sit higher than points for the 0-second and 20-second groups. An outlier might be a tester who moved more beads after the soak, which can happen because the second try benefits from practice.Answer: Accept the difference between the student's two class means.
The 60-second mean minus the 0-second mean should be a positive number in most classes. If it is not, ask students what might have hidden the effect, such as practice making the second trial faster for everyone.Answer: It controls for wet hands.
Wet fingers can slip on beads. Soaking the 0-second group in room-temperature water means the main difference between the groups is the cold, so any extra drop in the cold groups can be linked to temperature.Answer: Accept any reasonable line and a judgment that matches it.
A good line passes through the middle of the points, with about as many points above it as below it. Students should say that points close to the line mean a line is a useful model, while points spread far from it mean the line gives only a rough estimate.Answer: 0.1 beads per second.
Slope = (7 − 1) ÷ (60 − 0) = 6 ÷ 60 = 0.1. Each extra second of cold soaking goes with about one-tenth of a bead more drop.Answer: 5.5 beads. y = 0.1 × 45 + 1 = 4.5 + 1 = 5.5.
Answer: Cold water weakens the hands more the longer it cools them, so gripping the rope gets harder each minute.
Strong answers use the class trend: longer soaks went with larger drops in hand skill. A swimmer in cold water loses grip and finger control in the same way, so a rope that could be held at first may slip out of the hands later. Accept answers that also mention tired arms or the cold making it hard to swim.Answer: A life jacket.
A life jacket keeps the swimmer afloat even after the hands and arms stop working well, which the class data suggest can begin quickly. A plan to swim to shore depends on the arms and hands staying strong, and cold water weakens exactly those. Accept an answer that combines both, as long as it recognizes that the life jacket does not depend on muscle strength.
Differentiation
Support
Give these pairs a class table with the group means already calculated, so their work begins at the scatter plot. Provide graph paper with the axes drawn and labeled in steps of 10 seconds and 1 bead. For question 5, show the slope formula with the two points already placed in it.
Extension
Have students test whether the effect fades with rewarming by repeating the bead task 2 minutes after the soak and adding a column to their data table. Ask them to draft one sentence for a river safety sign below the dam, based on how fast hand skill dropped and how fast it returned.
ELL supports
Pre-teach "baseline," "soak," "drop," "association," and "outlier" with a labeled sketch of the two tubs and the bead dish. Provide sentence frames for the analysis: "As soak time increases, the drop in beads ___." and "A life jacket helps because ___."
Extensions
Students can compare their bead results with a simple skin-temperature reading, taken by holding the thermometer bulb in a closed fist for 30 seconds before and after the soak, if the teacher approves. A parks ranger or dam safety staff member can be invited to explain how signs, sirens, or posted release schedules warn people downstream of a dam that the river can rise quickly or run cold.