IB Physics IA idea · Motion, sport and play
IB Physics IA idea: which drag law fits a damped pendulum?
A pendulum with a big flat bob stops far faster than a compact one. Work out which drag model fits the decay.
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 frontal area of a card disc bob, from 4 cm² to 60 cm² in five sizes, change the number of swings a pendulum takes to fall to half its amplitude, and is the decay closer to linear or quadratic drag, using Tracker video?
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
- bob frontal area, 5 values from 4 to 60 cm^2 (flat card discs)
- You measure
- amplitude after N swings, and number of swings to fall to half amplitude
- You keep the same
- pendulum length
- bob mass
- initial amplitude
- string and pivot
Method
Hang card discs of different diameters from a fixed length of thread and film the decay with a phone and Tracker. Extract successive peak amplitudes.
Data analysis
Plot ln(amplitude) against time (straight line for linear drag) and the inverse amplitude against time (for quadratic drag). Fit both models, compare residuals, and discuss which regime applies. Report the decay constant against area.
How it works
You change Bob frontal area (cm²) and measure Amplitude after swings (degrees) in Pendulum with card disc bob, keeping Thread length, Bob mass, Release method the same. Expect a graph of Amplitude / ° against Time / s: The way it dies away tells you which kind of drag wins.
Make it yours
Change the bob shape, or the fluid by immersing a bob in a tank of water or oil.
Extended Essay angle
Model comparison and the effect of Reynolds number on the drag law give a deeper analysis.
Safety
Clamp firmly.
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