The spreadsheet uses a short piece of VBA (Visual Basic for Applications) code to loop through one minute's worth of oscillations and exactly reproduces the expected results. Although the theory is relatively straightforward, many learners find it more convincing to see the theory presented in this fashion.
3.2. Large amplitude oscillations
Videos of pendulum wave machines often show relatively large angle oscillations. This leads to a natural frequency that is a little different from that calculated using the formula above (and a motion that is not perfectly sinusoidal), but this can largely be compensated for by adjusting the string lengths until the frequency is as desired for a given amplitude. However, a problem arises as the pendula are damped and lose energy—their frequency will no longer be independent of amplitude and so the frequency shifts will not remain constant. Evidently the effects over one full set of cycles are relatively small, as seen in the various videos.
An investigation into large angle pendula shows that the frequency shift up to about 30° (0.5 radians) is about 1.5% [1]. It would appear that, over 60 cycles, this would add up to almost a whole cycle shift (60 × 1.5% = 90%). However, the important figure is not the difference in frequency from that of perfect SHM, but the difference from that of the final amplitude. In fact, the only difference to be concerned about is the difference in the amount of damping between pendula, as if they all damp together, their frequencies will shift together. The overall cycle time will be changed but not the visual effect. This is a good example of errors cancelling and nature conspiring to help us for once!
4. Conclusion
The pendulum wave machine is an appealing physics device that is easily explained, and helps students to understand the idea of phase, or more particularly frequency, as f = dphgr/dt. It can be used to introduce simple spreadsheet visualization of a problem—which is only possible once the problem is properly specified and set up—and to explore deeper questions such as the effect of large amplitude.
Acknowledgments
This paper was inspired by one of those classic examples of no question being too silly, asked by a former colleague, Gerry Leversha. The spreadsheet was improved by David May, former head of physics and ICT at St. Paul's School.
References
[1]
Johannessen K 2010 An approximate solution to the equation of motion for large-angle oscillations of the simple pendulum with initial velocity Eur. J. Phys. 31 511
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Biographies
Ken Zetie
Ken Zetie is head of science at St Paul's School in London, UK, a member of the editorial board of Physics Education, and an inveterate player with spreadsheets, toys and physics.