So-called "soap boats" have been known for decades and can be used to demonstrate the Marangoni effect. Inspired by recent scientific work, this paper reports an improved demonstration: a "soap boat 2.0". With this demonstration, a floating object (typically polystyrene foam) can be propelled along the water surface for up to several minutes. The direction of motion (straight, left-handed, or right-handed circles) can be influenced by the boat design. Three easy-to-make and effective boat designs are presented, and fifteen "fuels" have been considered with regard to safety, effectiveness, duration, ease of use, and availability. Some other possible variations of the demonstration are also proposed. The demonstration can be done with any age group, lasts for up to several minutes, and can be turned into a student project of varying length. Readily available materials (e.g., polystyrene foam) and benign household chemicals (e.g., ≥10% 2-propanol ("rubbing alcohol") in water) can be used.
wrote that he had found the Paper extremely explicit except for part of 0 9, in which the Author stated that 'the discrepancy previously noted between the earlier relaxation solution and experiments has been almost entirely removed by the present calculations'. Perhaps Dr Markland would explain whether or not this increased accuracy was caused solely by the use of finer nets than those which were practical without the aid of a computer. Could he also indicate the theory behind the extrapolation to determine the value of d, for an infinitely fine net? It appeared that if a correlation existed between the values of d, for various values of a it was unnecessary to use finer nets.12. The Paper was particularly interesting to Mr Acton as he had produced work concerned with a parallel problem: that of the torsional analysis of thick-walled prismatic members.' At first glance the problems did not appear to be in the least similar, but the governing equation of the torsion problem was Poisson's equation:a24 5 2 4 axa ay which might be compared with equation (5).13. The components of shear stress caused by torsion were given by:. . . . . (29) which was comparable to equation (6). 14. The solution of the torsion problem by the relaxation method was also relatively simple to programme for a digital computer. In the case of his work a Standard Telephones computer was used, and it was found that the programming itself was a most intriguing exercise. It had to be emphasized that the computer could take no decisions of its own accord, and all eventualities had therefore to be considered when writing the programme. It was surprising and chastening to discover the number of steps which were taken or omitted without consideration when performing routine calculations, and programming was a sure way to clarify any analytical procedure.Professor Ch. Jaeger (English Electric Co. Ltd, Rugby) wrote that the flow over a free overfall or broad-crested weir was one of the simplest which could be considered in open channel hydraulics. It had attracted the attention of many specialists, but the problems connected with it had nevertheless never been solved to their entire satisfaction. In 0 3 the Author stated clearly that he assumed the flow to be frictionless, and implicitly assumed that at the far upstream end of the flow all the velocities were parallel (horizontal) and identical. These simplifying assumptions made treatment by the potential flow theory easier. Results of the calculation were shown
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