Today's paper gave me the chance to refresh my knowledge about ebb and flow (falling and rising tide). We remember: The water gets elevated by the gravitational force of the moon. This created the tidal bulge (~54cm) just underside the moon. As moon and earth both circulate around a point (barycenter) which is not in the earth center, the resulting “wobble” movement of the earth creates another tidal bulge on the opposite side (see figure). Because of the 24h rotation of the earth, any point on the earth crosses both tidal bulges over the day (~12h tidal rhythm). The tidal high varies over the month, because the gravitational force of sun also creates a water bulge (~25cm) which, depending on the moon-sun-location, either enhance the moon tidal bulge (spring tides at new moon and full moon) or counteract with the moon tidal bulge (neap tides at half moon). The standard calculation of the tidal bulge height is quite easy but it is based on some simplifications. First, the calculations are for a earth without land, which is completely covered with water. Second, only the earth and the moon contribute to the gravitational potential for points near the surface of the earth. However, the tidal bulge itself also produces a gravitational potential which should be considered. How to calculate this additional potential and how it change the end result, can be found in the paper of Travis Norsen et al. (2017). As mentioned by the authors, the idea of gravitational self-interaction of the tidal bulge is not new. The equation was already published in 1775 by d’Alamber. “But since it seems to have been— inappropriately, we think, due to the size of the effect— largely forgotten in pedagogical discussions of the tides, we thought it was worth sharing our approach with educators and students in its original format.” (Norsen et al. 2017) The gravitational self-interaction of the Earth's tidal bulge.
Norsen, T., Dreese, M., & West, C. (2017). American Journal of Physics, 85(9), 663-669.
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Matrix isolation is an experimental technique in which guest particles (atoms, molecules, ions, etc.) are embedded in a matrix of unreactive host substrates. Embedded in the matrix, the guest particles are isolated from each other. Diffusion processes and chemical reactions between the guest particles are prevented (reactions between guest and host particles are possible). “The matrix isolation technique was first introduced in 1954 by Pimentel and co-workers, who used the technique for systematic studies of free radicals and other unstable or transient species.“ (http://www.ruhr-uni-bochum.de/oc2/matrix.html). Initially matrix isolation was used to describe the embedding of guest particles in any unreactive material, but today mainly gases in low temperature solids are used as host medium (https://en.wikipedia.org/wiki/Matrix_isolation). That is interesting, as the paper written by Anthony J. Rest in 1990 showed that “many low temperature media, which have been regarded as ‘infra dig’, can produce results which are analogous to those obtained using gas matrices”. "Infra dig matrix media."
Anthony J. Rest Journal of Molecular Structure222.1-2 (1990): 87-93. |
IdeaI love to increase my general science knowledge by reading papers from different fields of science. Here I share some of them. Archiv
März 2018
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