Investigating Terminal Velocity
Two Experiments That Make Drag Impossible to Ignore
Terminal velocity is often introduced with equations and free-body diagrams. These two experiments turn it into something students can see, measure, and explain — with clean data and a memorable visual payoff.
Both experiments isolate shape and surface area while keeping mass constant.
Experiment 1 – Same Mass, Different Shapes (Water Tube)
The Question
If mass is the same, does shape alone change terminal velocity?
Apparatus
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2 m transparent vertical tube filled with water
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PASCO rotation sensor
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Thin, low-stretch line
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Small masses of identical mass but different shapes
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sphere
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cylinder
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flat disc / paddle shape
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Data logger (PASCO Capstone)
Method
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Attach the first mass to the line and zero the sensor.
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Release it gently into the water column.
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Record velocity vs time.
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Repeat for each shape.
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Plot velocity–time graphs on the same axes.
What Students See
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All objects start by accelerating.
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Each reaches a constant speed.
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Terminal velocity varies significantly with shape, even though mass is identical.
A sphere reaches the highest terminal velocity. Flat shapes reach it fastest — and at a much lower value.
Physics Link
At terminal velocity:
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Weight = Drag
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Acceleration = 0
Drag depends on:
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fluid density
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speed²
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cross-sectional area and drag coefficient
Same mass ≠ same motion.
Experiment 2 – Open vs Closed Umbrella (Air)
The Question
Does surface area dominate motion through air?
Apparatus
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Two identical umbrellas
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High window / balcony (with clear drop zone)
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Stopwatch or video timing (optional)
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Optional comparison to water-tube data
Method
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Drop the closed umbrella and observe the fall.
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Drop the open umbrella from the same height.
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Repeat for consistency.
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Discuss qualitatively or time using video playback.
What Students See
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Closed umbrella: rapid acceleration, short fall time.
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Open umbrella: slow, steady descent at much lower terminal velocity.
Even without sensors, the contrast is unmistakable.
Bringing the Two Experiments Together
| Feature | Experiment 1 | Experiment 2 |
|---|---|---|
| Fluid | Water | Air |
| Measurement | Quantitative | Qualitative / timing |
| Variable | Shape | Surface area |
| Key idea | Drag coefficient | Cross-sectional area |
Together, they show:
Terminal velocity is not about mass — it’s about drag.
Common Misconceptions Tackled
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❌ Heavier objects always fall faster
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❌ Terminal velocity only applies to skydivers
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❌ Acceleration is constant during a fall
These experiments dismantle all three.
Why This Works Brilliantly in Teaching
✔ Clear cause-and-effect
✔ Safe and repeatable
✔ Excellent graphs for exam questions
✔ Highly memorable (students remember umbrellas!)
Perfect for GCSE Forces and A-level Mechanics.
https://youtu.be/fB1D-JQMBHg?si=uQk30WuXvBjIRHyM
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