Bench experiment, five pressures, 100 decay runs

Physics research poster example

Air damping of a torsion pendulum measured at five chamber pressures, laid out as an A0 portrait board where one hero figure takes the whole upper half. Four decay curves on one set of axes carry the result, and the columns underneath say how the fit was tested.

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EU
Air damping of a torsion pendulum from atmosphere to one kilopascal: one hundred decay runs
K. Ostrowski ¹, A. Bhattacharya ¹
1 Department of Physics, Ellisbrook University
Abstract
A torsion pendulum is the standard teaching demonstration of damped oscillation, and the standard treatment assumes damping is proportional to angular velocity with a coefficient that does not depend on amplitude. We tested that assumption by measuring free decay at five chamber pressures from 100 kPa down to 1 kPa, twenty runs at each pressure, one hundred runs in total, with angle recorded optically at 500 Hz. Ring down time, defined as the time for the amplitude envelope to fall to one over e of its starting value, rose from 41 seconds at atmospheric pressure to 318 seconds at 1 kPa. A single exponential fits every decay to a residual under 3% of full scale, so within this range the linear viscous model holds and the residual damping at 1 kPa is dominated by the suspension wire rather than by the air. The apparatus is a teaching set with one addition, a vacuum chamber and a pump, and the whole measurement takes an afternoon.
1 kPa
10 kPa
100 kPa
30 kPa
0
100
200
300
0255075100
Figure 1. Amplitude envelope as a percentage of the starting amplitude, four of the five pressures.
Source: 20 free decay runs at each pressure, optical angle sensor at 500 Hz.
METHODS
Apparatus. Teaching torsion pendulum on a 0.2 mm steel wire inside a 40 litre vacuum chamber.
Pressures. Five settings: 100, 30, 10, 3 and 1 kPa, read on a capacitance gauge to 2%.
Runs. Twenty free decay runs at each pressure, released from the same 12 degree amplitude.
Recording. Optical angle sensor at 500 Hz, envelope extracted from successive turning points.
Fitting. Single exponential to the envelope, residual reported as a percentage of full scale.
REFERENCES
1.Bhattacharya, A. (2025) Envelope extraction from turning points in low rate angle data. Review of Teaching Instruments, 12(1), 18 to 33.
2.Kowalczyk, R. and Ostrowski, K. (2024) Linear and quadratic drag in low pressure torsion pendulums. European Journal of Physics Practice, 45(3), 210 to 229.
3.Nakashima, E. (2026) Wire losses as the damping floor. Ellisbrook Physics Notes 2026-01.
Results
Ring down time was 41 s at 100 kPa, 62 s at 30 kPa, 88 s at 10 kPa, 152 s at 3 kPa and 318 s at 1 kPa. Run to run spread was under 4% at every pressure except 1 kPa, where it reached 7% because the decay outlasts the temperature stability of the chamber. Plotted against the reciprocal of pressure the five ring down times fall on a straight line with an intercept of 470 s, which is the wire.
Model fit
A single exponential fits every one of the hundred decays with a residual under 3% of full scale, and adding a quadratic drag term does not reduce it. The linear viscous model is therefore sufficient over this pressure range and this amplitude, which is what the teaching treatment assumes. The interesting part is the intercept rather than the slope: extrapolated to zero pressure the pendulum would still ring down in about 470 s, so at 1 kPa two thirds of the remaining loss is already in the wire and not in the air.
318 s
ring down time at one kilopascal
Against 41 s at 100 kPa, twenty runs at each of five pressures
Conclusion
Damping falls with pressure, stays linear at every pressure, and the wire sets the floor.
Ring down 41 s at 100 kPa, 318 s at 1 kPa
Single exponential fits all 100 runs under 3% residual
Residual damping at 1 kPa comes from the wire
Teaching apparatus plus a chamber and pump, one afternoon
Apparatus notes
The chamber, pump and gauge were borrowed from the second year laboratory and the optical sensor was built from a spare photodiode board. The 3 kPa curve is left out of the figure so four traces stay readable; its ring down time of 152 s is in the results text. Thanks to the second year laboratory technicians for the loan and for pumping the chamber down four times in one afternoon.

Block by block

What each block on the board is for, in the order a reader walks it.

Title band, two authors and a QR code
The title gives the range and the effort: air damping of a torsion pendulum from atmosphere to one kilopascal, one hundred decay runs. Two authors share one department affiliation, with the QR code at the right of the band.
Abstract
A wide block stating the assumption under test, the five pressures, the twenty runs at each, the optical recording at 500 hertz, the ring down times of 41 and 318 seconds, the under 3% residual, and the note that the apparatus is a teaching set plus a chamber and pump.
Hero figure
Amplitude envelope as a percentage of starting amplitude, plotted at four time points for four of the five pressures. The 100 kPa curve falls to near zero within a hundred seconds while the 1 kPa curve is still near 39% at three hundred.
Methods as a labelled protocol
Five entries: a teaching torsion pendulum on a 0.2 millimetre steel wire in a 40 litre chamber, the five pressures read on a capacitance gauge to 2%, twenty runs released from the same 12 degree amplitude, optical angle recording at 500 hertz, and a single exponential envelope fit.
Results and the headline number
Ring down times of 41, 62, 88, 152 and 318 seconds, with run to run spread under 4% except at 1 kPa. A key number block reads 318 seconds, labelled as the ring down time at one kilopascal against 41 at 100 kPa.
Model fit
A single exponential fits all one hundred decays under 3% of full scale and a quadratic drag term does not improve it, so the linear viscous model is sufficient over this range. The intercept at zero pressure is 470 seconds, which is the wire.
Conclusion, apparatus notes and references
A one line conclusion and four checked points cover the ring down range, the fit, the wire floor and the cost of the apparatus. The apparatus notes record the borrowed chamber and the omitted 3 kPa curve, above three numbered references.

What makes this board work

Four curves on one set of axes

Plotting the amplitude envelope for four pressures together lets a reader see both the ordering and the shape without comparing four small panels. The fifth pressure is left off the figure and reported in the text, which the acknowledgements block says plainly.

The model test is a number

A single exponential fits all one hundred decays to a residual under 3% of full scale, and adding a quadratic drag term does not reduce it. That is a testable claim, unlike saying the fit was good.

The intercept is the interesting result

Plotted against reciprocal pressure the five ring down times give an intercept of 470 seconds, so at 1 kPa two thirds of the loss is already in the wire. The board makes that the discussion rather than burying it under the slope.

Questions people ask

What goes on a physics research poster?

The apparatus, the parameter varied and its range, the number of runs, how the measurement was extracted, one figure showing the behaviour, the model tested and the residual. A fit described as good without a residual figure is not a result a reader can use.

How many curves can one poster figure hold?

Four is comfortable because the series colours cycle through four distinct tokens before repeating. A fifth curve takes the first colour again and two lines become indistinguishable, so this board plots four pressures and reports the fifth in the results text.

How big should the hero figure be?

On this layout it takes the full width and roughly the top half of the board below the title, which at A0 portrait is about 1552 by 900 on the working canvas. That is enough for four labelled curves and an axis a person can read from a metre.

What text size works at a poster session?

Body text here is near 24 pixels on the 1656 by 2340 canvas, about 12 millimetres printed, readable at a metre. Axis numbers are smaller by design, so keep the tick labels to a few characters and put units in the caption rather than on every tick.

Can I print an A0 physics poster at 300 DPI?

The export doubles the working canvas, giving 3312 by 4680 pixels at A0 portrait, which is roughly 100 DPI at full size. That is the standard working resolution for a poster read from a metre away. Bleed, crop marks and CMYK are not produced.

How do I adapt this to a different experiment?

Retype the title, abstract and the five method entries, then edit the figure series and categories directly. Each curve is a named series with its own values, so adding or removing a pressure is a data change. The chart can also be switched between ten chart types.

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