Where must pennies sit to balance a ruler?
- 👶 Small parts
My hypothesis
If I change how many pennies are in the stack on the right, then how far from the pencil the stack must sit to balance the ruler will ______, because ______.
You fill in the blanks. Think about how a see-saw balances when one side has more weight than the other. A guess is a good because: the test finds out.
Change, measure, keep the same
CHANGE
how many pennies are in the stack on the right: 2 pennies, 4 pennies, 6 pennies, 8 pennies
MEASURE
how far from the pencil the stack must sit to balance the ruler, in cm
KEEP THE SAME
- the same ruler on the same pencil
- four pennies taped 6 cm to the left of the pencil
- the ruler sitting level before you measure
Scientists call the thing you change the independent variable and the thing you measure the dependent variable. The things you keep the same are the controls: if one of them changed too, you wouldn't know which change made the difference. Try each of the 4 levels 3 times.
What you need
- Pennies: about 25
- A pencil
- Masking tape, to mark lines
- 🩹 Tape
- 📏 A ruler
Steps
- Tape a pencil to a table so it cannot roll.
- Slide the ruler across the pencil until it sits level, and write down the number over the pencil.
- Tape pennies into stacks of 2, 4, 6 and 8, and one more stack of 4.
- Tape the extra 4-penny stack to the ruler, 6 cm to the left of the pencil.
- Set the 2-penny stack on the right side and slide it until the ruler sits level.
- Read the number under the stack's middle and take away the pencil's number: that's the distance.
- Write it in your table under Try 1, then lift the stack off and find the balance two more times.
- Do three tries with the 4-penny stack, then the 6-penny stack, then the 8-penny stack.
- Find the mean for each stack and draw a line graph.
Every step is one you can do yourself, with a grown-up nearby. Write each number down straight away, in the table below, so nothing is lost.
Your results table
Name ______________________ Fair day ____________
Question: Where must pennies sit to balance a ruler?
I changed: Pennies on the right I measured: how far from the pencil the stack must sit to balance the ruler (cm)
| Pennies on the right | Try 1 (cm) | Try 2 (cm) | Try 3 (cm) | Mean |
|---|---|---|---|---|
| 2 pennies | ||||
| 4 pennies | ||||
| 6 pennies | ||||
| 8 pennies |
MY GRAPH (a line graph: pennies on the right along the bottom, cm up the side)
What I noticed: ______________________________________________
From 4th grade, add the tries for each level and divide by how many there were: that's the mean, and it goes in the last column. Then draw a line graph, because the thing you change is a number that goes up.
Name ______________________ Fair day ____________
Question: Where must pennies sit to balance a ruler?
If I change how many pennies are in the stack on the right, then how far from the pencil the stack must sit to balance the ruler will ________________, because _____________________________________________
| I CHANGE | I MEASURE | I KEEP THE SAME |
|---|---|---|
| Pennies on the right: 2 pennies, 4 pennies, 6 pennies, 8 pennies | how far from the pencil the stack must sit to balance the ruler (cm) |
|
What I think will happen, in my own words:
Name ______________________ Fair day ____________
Question: Where must pennies sit to balance a ruler?
| Date | What I did | What I saw | What I think |
|---|---|---|---|
Name ______________________ Fair day ____________
Question: Where must pennies sit to balance a ruler?
| Date | What I did | What I saw | What I think |
|---|---|---|---|
Name ______________________ Fair day ____________
Question: Where must pennies sit to balance a ruler?
| Date | What I did | What I saw | What I think |
|---|---|---|---|
Name ______________________ Fair day ____________
Question: Where must pennies sit to balance a ruler?
My hypothesis was ☐ right ☐ partly right ☐ not right.
I know because my numbers show:
What surprised me, or what might have gone wrong (7th and up):
Next time I would test:
Idea: Change a second thing: move the left stack to 4 cm and to 8 cm, repeat all four tests for each, and draw the lines on one graph.
Why it works
A ruler on a pencil is a lever, and the pencil is its pivot. A weight turns the ruler more the farther it sits from the pivot, so a heavier stack has to sit nearer to balance the same load. When the ruler sits level, weight times distance is the same on both sides.
Going further (7th and up): Change a second thing: move the left stack to 4 cm and to 8 cm, repeat all four tests for each, and draw the lines on one graph.
Board layout
- ❓ Question (10 × 8 in): Where must pennies sit to balance a ruler?
- 💭 Hypothesis (10 × 9 in): If I change how many pennies are in the stack on the right, then how far from the pencil the stack must sit to balance the ruler will ______, because ______.
- 🧺 Materials (10 × 12 in): 3 things: see your list.
- 🪜 Steps (22 × 9 in): 9 steps, three tries at each level.
- 📊 Results (22 × 16 in): Your table (Pennies on the right down the side, tries across) and your line graph.
- ✅ Conclusion (10 × 14 in): My hypothesis was right / partly right / not right. I know because…
Materials to list: Pennies; A pencil; Masking tape, to mark lines.
Bring photos or drawings of your test. Nothing wet, hot, powered or alive on the table.
This project at your grade
| 4th–6th grade | Tries: three at each level. The table: units in the header and a mean column. The graph: a bar graph, or a line for a number that grows. The conclusion: right, partly or not, and the numbers that show it. |
|---|---|
| 7th grade and up | Tries: three or more at each level. The table: a mean for every level of both tests. The graph: a graph with two lines or two sets of bars. The conclusion: the numbers, what might have gone wrong, and a next question. Second variable: move the left stack to 4 cm and to 8 cm, repeat all four tests for each, and draw the lines on one graph. |
Every grade this project fits works out a mean. See every project for 4th grade, or print a blank results table.
What a judge will ask
- What did you change, and what did you measure? I changed how many pennies are in the stack on the right and measured how far from the pencil the stack must sit to balance the ruler, in cm.
- What did you keep the same, and why does that matter? I kept the same ruler on the same pencil and four pennies taped 6 cm to the left of the pencil the same, so the only difference was how many pennies are in the stack on the right.
- How many times did you test it? 3 times at each of my 4 levels.
- If you did it again, what would you change or test next? I'd move the left stack to 4 cm and to 8 cm, repeat all four tests for each, and draw the lines on one graph.
Make it yours
Keep this project on this device and Peabody counts down to your fair day, with a plan for each day. Nothing is sent anywhere.
Try next
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