Teach lesson
Newton's Cradle: momentum, energy, and real evidence
Students use the remote Newton's Cradle lab to observe 1-, 2-, 3-, and 4-ball releases, explain the pattern with momentum and kinetic energy, and distinguish an ideal model from real video evidence.
Learning Outcomes
Observe what happens when 1, 2, 3, and 4 balls are released in a real remote Newton's Cradle.
Record how many balls leave after the collision and rate confidence in the observation.
Explain why the same-number pattern requires both momentum and kinetic energy reasoning.
Identify conditions that make the cradle approximate an ideal model.
Distinguish real-video evidence, physical model, and idealization.
Write a CER conclusion with an honest limitation.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
Predict the pattern before opening the lab
8 min
Newton's Cradle looks simple: lift some balls, release them, and other balls leave from the opposite side. But a strong explanation cannot stop at "that is just what happens." It must separate three things:
- observation: how many balls are released and how many leave;
- ideal model: equal balls, good alignment, and nearly elastic collisions;
- real video: blur, small losses, setup hand, and oscillations that fade out.
Real video from the remote lab
The activity starts with a concrete observation: count balls in real video, not in a perfect drawing.
Before opening the lab, predict what should happen when 1, 2, 3, and 4 balls are released from the same side. Add one sentence about why the real video might not look as clean as the ideal model.
Open the lab and observe like an investigator
10 min
Use the remote lab as evidence. You do not need exact heights in centimetres. The important work is to record what the video can support clearly.
Workflow
The observation table includes confidence because the evidence comes from real video.
Open the Newton's Cradle lab
Open the lab from TEACH.
Run the 1-, 2-, 3-, and 4-ball actions from the TEACH-launched lab.
For each action, record how many balls are released and how many balls you see separate on the opposite side after the collision.
If the video has blur, a setup hand, or motion that is difficult to count, do not invent precision: write a confidence note.
Do not measure exact heights unless the interface provides a clear scale and your teacher asks for that as an extension.
Write your observation plan in two or three sentences. State what you will count, how you will compare the four actions, and how you will record uncertainty if a video is not clear.
Record the outgoing-ball pattern
16 min
Complete the table with your observations before you compare them with the ideal model.
Here, confidence means how sure you are that the video supports your count. Use high for a clear, easy count; use medium or low when blur, a setup hand, rebound, damping, overlap, or counting difficulty makes the row less certain.
Observation scaffold
Use the scaffold to record what you saw. Keep the ideal model separate until after the observation table is complete.
Newton's Cradle observations
Use the 1-, 2-, 3-, and 4-ball actions from the TEACH-launched lab. For confidence, write high, medium, or low. In the evidence note, explain why that row is clear or uncertain.
| Released balls | Observed outgoing | Confidence (high/medium/low) | Evidence note |
|---|---|---|---|
Before the conclusion, write down one real limitation from the video: blur, setup hand, friction, damping, misalignment, or counting difficulty.
Summarize the quality of your observations. Mention at least two rows from the table and explain which row has higher or lower confidence.
Based on your table, does the lab support the pattern that approximately the same number of balls leaves as the number released? Cite data from at least three rows.
Explain the pattern with momentum and energy
14 min
Now connect your observations to the physical model. In a nearly elastic collision between equal-mass balls, the pattern is not explained by momentum alone. Kinetic energy matters too.
Momentum and kinetic energy
One ball at twice the speed may seem compatible with momentum, but not with kinetic energy.
Two quantities must fit
p = mv
\qquad
K = \frac{1}{2}mv^2
For one ball, doubling speed would make its kinetic energy four times larger. In the two-ball case, one ball leaving at twice the speed would carry twice the total kinetic energy of two equal balls leaving at the original speed, so momentum alone is not enough.
After collecting your observations, which pattern is most reasonable for steel balls that are approximately equal in mass and well aligned?
If two equal balls are released, why is it not enough to say "one ball leaves at twice the speed"?
Explain in your own words why the same-number pattern is compatible with conserving momentum and kinetic energy. Use the 2-ball case as your example, and explain what would go wrong if one ball left at twice the speed.
Name three conditions that help the cradle behave like the ideal model. Briefly explain what could happen if one of them fails.
Treat real video as evidence, not perfection
10 min
A real remote lab is not an ideal simulation. That is useful if you handle it well: you can discuss evidence, confidence, and limits.
Choose one real limitation you observed or that is visible in the frames. Examples include blur, setup hand, damped oscillation, friction, sound/heat, misalignment, or counting difficulty. Explain how it affects your confidence without invalidating the whole activity.
Optional annotated frame
Optional if your teacher requests it: attach or describe one lab frame with marks showing the balls you counted and a confidence note.
CER conclusion: evidence and model
12 min
Your conclusion should avoid two extremes:
- it is not enough to say "the same number leaves because I memorized it";
- you should not demand precision that the video does not provide.
Write a complete CER conclusion:
- Claim: the main pattern supported by the lab.
- Evidence: cite at least three rows from your table.
- Reasoning: connect the pattern to momentum and kinetic energy.
- Limitation: explain one real limitation of the video or ideal model.
Optional extension: compare class observations if assigned
12 min
If several groups observed different iterations, compare whether lower-confidence rows are consistent or depend on the specific video.
Class observation comparison
Each group contributes one row for the action it found most interesting or ambiguous. Use high, medium, or low confidence and name the main reason.
| Group | Released balls | Observed outgoing balls | Confidence (high/medium/low) | Main limitation |
|---|---|---|---|---|