Lesson 3 Grades 6–8
Stability: the balance point and the push point
- Timeabout 60 minutes
- WorksheetW3 Swing test
- Rocket BuilderBeginner mode
Worksheets (PDF) Open Rocket Builder
Standards
| Code | In plain words |
|---|---|
| MS-ETS1-3 | Analyze test data from several designs to find the best features of each. |
| MS-PS2-2 | Forces (here, the air on the fins) change an object's motion. |
| 6.EE.A.2 (math) | Evaluate a formula: score = (CP − CG) ÷ body width. |
Practice: analyzing and interpreting data. Crosscutting concept: stability and change.
I can…
- I can find a rocket's balance point (CG) and estimate its push point (CP).
- I can explain why the CG must be in front of the CP.
- I can work out a stability score in calibers and say whether it's safe (1–2 cal is ideal).
Words to know
- Centre of gravity (CG), balance point Rocket Builder's blue dot.
- Centre of pressure (CP), push point where the air's push acts on average. The red dot.
- Caliber (cal) one body width, used as a ruler.
- Stability margin (score) how far the CP is behind the CG, in calibers.
Materials
Paper-towel tube, cardstock nose and fins, modeling clay, 1.5 m of string, scrap cardboard, a ruler, W3.
Steps
1Teach10 min
Read aloudFins are a rocket's feathers. If a rocket tilts, air hits its side. When the push point is behind the balance point, the air swings the tail back in line, like a weathervane. If the push point is in front, the air swings the nose the wrong way and the rocket tumbles.
Rocket Builder's score: under 1 cal too wobbly (no download), 1–2 ideal, 2–3 fine, over 3 so stable it turns into the wind. The app checks it with the heaviest motor loaded, the worst case. To fix a low score: bigger or swept fins, a longer body, or nose weight.
2Measure15 min
- Build a test rocket (tube, cone nose, 3–4 fins). Balance it on a finger: mark the CG (blue). Measure from the nose tip.
- Trace the rocket's side view on cardboard, cut it out and balance the cut-out: that's a cautious estimate of the CP (red). Mark it.
- Score = (CP − CG) ÷ body width.
3Swing-test demo20 min
Outdoors, nobody within 3 m. Tie the string at the CG; swing the rocket in a circle; does the nose lead? Test A no fins, B fins, C fins + nose clay, D fins + tail clay. Re-find the CG each time. Record on W3.
4Rocket Builder10 min
Build a rocket. Find the blue and red dots and the score. Change only the fin shape and rebuild. Which fins give the highest score?
5Wrap up5 min
Which version was best, and what did the data show about why?
Check for understanding
- Which must be in front: CG or CP?
- Body 2 cm wide; CP is 3 cm behind the CG. Score?
- Is 0.5 cal OK to fly?
- Name two ways to raise the score.
Answer key
- The CG (blue) in front of the CP (red).
- 3 ÷ 2 = 1.5 cal.
- No: wobbly; Rocket Builder won't let you download it.
- Bigger or swept-back fins, a longer body, weight in the nose.
Teacher notes
- The swing test is a demonstration, not proof. It can give wrong answers (string position, swing speed). For the real rocket, the go/no-go check is the balance point: with a motor in, the measured CG must sit in front of Rocket Builder's CP by at least one body width (Lesson 8).
- What to expect: A tumbles, C is steady, D wobbles or tumbles.
- Extension: a Rocket Builder rocket with CG 253 mm and CP 289 mm from the tip, body 21.4 mm: (289 − 253) ÷ 21.4 ≈ 1.7 cal. Look up the Barrowman method, the hand calculation the app uses.
- Further resources: Apogee Components' free lesson Fins and Finish Lines (apogeerockets.com/downloads/PDFs/58-3.pdf).
- Nose weight setting: Advanced mode has Nose weight (clay), in grams. Add 5 g at a time and rebuild: the CG moves forward and the score rises, but the predicted altitude drops. A good trade-off discussion.