IB Physics IA idea · Fluids and flow
IB Physics IA idea: ball size and terminal speed in syrup
Sink a ball bearing into a thick liquid and it quickly settles to a steady speed.
These ideas are starting points, not an IA to copy. Never copy an idea blindly: make the question, method and analysis your own, and check your plan with your teacher.
Example research question
How does the terminal speed of steel ball bearings in glycerol depend on radius from 1.0 mm to 2.5 mm in six sizes, filmed in a tall tube, and what viscosity follows from v against r²?
Change the system, the range or the method so the question is yours. Check it against the research question checklist in our Physics IA guide.
Variables
- You change
- ball radius, 1.0 to 2.5 mm in 6 sizes
- You measure
- terminal speed, 10 to 70 mm/s
- You keep the same
- liquid (glycerol)
- temperature
- tube diameter (at least 10 times the ball)
- ball material
Method
Drop ball bearings of several sizes in a tall tube of glycerol and film their fall against a scale; use the final slow section for the speed. Repeat 5 times.
Data analysis
Stokes: v = 2 r^2 (rho_s - rho_f) g/(9 eta). Plot v against r^2; the gradient gives the viscosity. Check the Reynolds number and apply the wall correction.
How it works
You change Ball radius (mm) and measure Terminal speed (mm s⁻¹) in Ball falling in glycerol, keeping Liquid temperature, Ball material, Tube width the same. Expect a graph of Terminal speed / mm s⁻¹ against Ball radius r / mm: Bigger balls fall much faster.
Make it yours
Warm or cool the liquid and track viscosity against temperature.
Extended Essay angle
Wall effects and finite-Reynolds corrections to Stokes' law.
Safety
Mop up glycerol spills; slippery.
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