When a metal disc with a hole in the center is placed in a vessel of water, it sinks. But if a stream of water is directed vertically into the center of the disk, it can be kept afloat. The study and explanation of this phenomenon was provided by SESC NSU 11th grade student Maria Buda when she conducted an experiment to test why a disk placed under a jet of water remains on the surface of the liquid. Her scientific advisors were Yury Bashkatov, Candidate of Physical and Mathematical Sciences, and Associate Professor at the NSU Department of Physics and the SESC NSU together with Nikita Sharov, a graduate of SESC and former participant in the Tournament of Young Physicists.
Buda described her work,
My research was quite active throughout the academic year. It started with observing the phenomenon, then I became acquainted with the literature and studied the theory. The result of my work is a model based on theoretical and empirical calculations that predicts the conditions under which a disk will be unsinkable.
During the research, an experimental setup was created that makes it possible to vary the water stream. For the experiment, disks of various sizes were made from different materials. This helped the formation on the disk of such a phenomenon as a hydraulic jump (a sharp increase in the water level during the transition of a liquid from a calm state to stormy) so its parameters could be studied. Also, based on the force of the water stream and taking into account the influence of the hole, it was possible to create a theoretical prediction of the parameters of a disk floating under given experimental conditions. In this case, the model corresponds well with experimental results.
As the study showed, the main factor that ensures the floating of the disk is the increase in liquid pressure under the disk due to the additional excess pressure of the liquid column. This overpressure results from a hydraulic jump. The action of this pressure on the lower surface of the disk makes it possible to compensate for the force of gravity and the dynamic force from the jet falling from above. The role of the hole is to reduce the effective flow rate of the jet, which was also taken into account in the solution.
Buda continues,
There were quite a few interesting problems at the Tournament this year but it was the problem of the unsinkable disk that immediately attracted me. This is probably due to the fact that in previous Tournament seasons I also solved problems related to hydrodynamics. When choosing a problem, I also relied on the desire to understand this area better.
