ROCKET BALLOON WITH WIRELESS ACCELEROMETER

Published: Sep-2024 | Category: Fun With Science

Turn a classroom favourite into a real physics investigation. A balloon rocket whizzing along a line always raises a smile. Add a Data Harvest Wireless 3‑Axis Accelerometer and that fun becomes a rich, curriculum‑ready experiment: students see the launch and measure the physics.

Sensor: Wireless Accelerometer 3‑axis

Learning objectives

• Relate thrust, drag and mass to measured acceleration (Newton’s 2nd law).
• Interpret acceleration–time data: thrust phase, coast/slowdown, and stop.
• Compare runs to explain how balloon size, line angle, line friction and added mass change the motion.
• Work with vectors by reading x/y/z axes or by deriving the resultant acceleration.

Apparatus

• Data Harvest Wireless 3‑Axis Accelerometer (with EasySense software).
• One long, smooth line: fishing line or taut nylon string.
• Two retort stands (or wall anchors) with bosses and clamps to tension the line.
• Short piece of straw (the line threads through the straw).
• Balloons (identical type if comparing fill levels).
• Adhesive tape (to mount the sensor to the balloon or straw carriage).
• Ruler or measuring tape (line length, balloon circumference).
• Protractor or digital angle gauge (for inclined runs).
• Small scales (optional, to estimate total mass).
• Eye protection; latex‑free balloons if required.

Setup (5–10 minutes)

1. Rig the line between stands, roughly chest height. Pull it tight. For extension work, set a small angle (for example, +5° or -10°) using a protractor.
2. Thread the straw onto the line. Tape the balloon to the straw (standard method) or tape the sensor and balloon to a lightweight paper carriage taped to the straw.
3. Mount the sensor so that one axis points along the line (the “along‑track” axis). Keep the tape light and symmetrical.
4. Connect and zero: pair the sensor, place it still for a moment and zero/offset within EasySense.
5. Sampling settings: start with about 100 Hz. Set a total capture time of 5–10 seconds.

Teacher tip: If you’re unsure about the best range/sensitivity, begin at a mid‑range, do one test run, and adjust to avoid clipping the peak.

Method

1. Mark balloon fill levels (for example, Small, Medium, Full). Use a strip of paper to measure circumference for consistency.
2. Inflate and hold the balloon nozzle shut. Keep the straw centred on the line.
3. Arm the recording in EasySense and release. Do not push.
4. Repeat at least three runs per condition for reliable comparisons.
5. Change one variable at a time (fill level, then line angle, then added mass, then line type, and so on).

What the data shows

• Thrust phase: a sharp positive acceleration spike along the line as air rushes out.
• Wobble and twist: small, rapid oscillations on the lateral axes (y and z) as the balloon yaws or pitches; the along‑track axis may show ripples too.
• Thrust ends: acceleration crosses through zero, then becomes negative as drag (and any line friction or downslope weight component) slows the rocket.
• Stop: brief spikes as the balloon shudders to a halt.

For cleaner comparison across runs, calculate resultant acceleration:

a_resultant = sqrt(ax^2 + ay^2 + az^2)

Data analysis

1. Annotate phases on the acceleration–time graph: thrust, coast/slowdown, stop: label in EasySense.
2. Record the peak acceleration during thrust for each balloon size.
3. Compare conditions: make a bar chart of peak acceleration versus balloon size; and a line chart of peak acceleration versus line angle.
4. Optional smoothing: apply a light smoothing in EasySense.
5. Extension: integrate the along‑track acceleration to estimate velocity–time (expect some drift).
6. Forces: discuss Fnet = m a. On an incline, include mg sinθ
   

Explain the science

• Newton’s 3rd law (action–reaction): air rushing out pushes backwards; the balloon is pushed forwards with equal and opposite force (thrust).
• Newton’s 2nd law: net force along the line causes acceleration (F = m a). Bigger thrust or smaller mass gives bigger acceleration.
• Drag: air resistance grows with speed and opposes motion.
• After thrust ends, drag (and friction, or mg sinθ on an incline) dominate, so acceleration becomes negative until the rocket stops.

Extensions and adjustments

A - Balloon fill level (thrust and impulse)

• Runs at Small, Medium and Full. Predict: fuller gives larger thrust spike and longer thrust duration.
• Plot peak acceleration and time above a threshold (for example, a > 0.5 m s⁻²) versus fill level.

B - Angle of the line (components of weight)

• Test 0°, 5° and 10° upslope and downslope.
• Predict: upslope adds mg sinθ opposing motion, reducing peak acceleration; downslope does the opposite after thrust ends.

C - Line and carriage friction (fair tests)

• Compare cotton string with fishing line; bare straw with PTFE‑lined straw.
• Expect smoother lines to give higher peak acceleration and longer runs.

D - Jet size (nozzle restriction)

• Narrow the nozzle slightly.
• Explore short, strong thrust (wide jet) versus long, weaker thrust (narrow jet).

E - Stability and fins

• Add a light card fin or use a guided cradle to reduce yaw.
• Compare lateral axis noise before and after.

F - Resultant versus along‑track

• Decide whether resultant acceleration or along‑track acceleration is the better single metric.

Suggested questions

• Where on the graph can you prove that thrust is present? Where does it end?
• Why does a fuller balloon change both the peak and the duration of acceleration?
• On a 5° incline, what extra term appears in the force equation along the track?
• Why might two runs with the same balloon size give different peaks?
• Which is the most reliable single number to compare runs: peak acceleration, time‑to‑zero, or area under the positive part of the acceleration curve?

Troubleshooting

• Flat, noisy traces: increase sample rate or check axis alignment.
• Clipped peaks: reduce sensor sensitivity or pick a higher g‑range.
• Wild lateral spikes: reduce wobble (better alignment, lighter tape, a stabilising cradle).
• Short runs: tighten the line; switch to fishing line; smooth straw edges.

Safety notes

• Wear eye protection with taut lines and popping balloons.
• Consider latex allergies; provide latex‑free alternatives.
• Do not over‑inflate; clear the run‑out area; keep the line above eye level.

Curriculum links

• Forces and motion: Newton’s laws, F = m a, components of weight.
• Graphs and data: acceleration–time, smoothing, averages, uncertainties.
• Vectors: axes, resultants, components, interpretation.
• Enquiry skills: variables, fair tests, repeatability, reliability, evaluation.

Simple results table (template)

Teacher wrap‑up

Students already love the rocket balloon. With the Data Harvest Wireless 3‑Axis Accelerometer, they can prove what is happening: thrust causes a big positive acceleration, then, once the air is gone drag and friction bring the rocket to rest. By changing one variable at a time (fill level, angle, mass, friction) and comparing the measured effect, they build a durable understanding of force, motion and fair testing.