PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Publ

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PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Public Information Number 150 November 5, 1993 CHAINS OF CRATERS ON CALLISTO AND GANYMEDE, two of Jupiter's moons, are now explained as being mostly due to split comets like Comet Shoemaker-Levy, which has broken into a chain of 22 fragments and is headed for a smashup with Jupiter in July 1994. Jay Melosh of the University of Arizona and Paul Schenk of the Lunar and Planetary Institute cite as evidence the fact that nearly all of Callisto's crater chains are on the Jupiter-facing hemisphere. In a separate paper, Melosh and James Scotti use a tidal-breakup model to calculate the size of the Shoemaker-Levy parent comet as being about 2 km. (Nature, 21 Oct. 1993.) Hubble Space Telescope pictures of the fragments suggest a size more like 3-4 km. The uncertainty in the size estimates translates into an uncertainty factor of 1000 for the energy of the collision. In any case, the ensuing explosions will cause Jupiter to ring, which might provide some rare seismological information about the planet's interior. (Nature, 28 Oct. 1993.) Study of reverberating waves in Jupiter's atmosphere should allow scientists to sharpen their views on the nature of the Great Red Spot. (Science, 22 Oct.) HINGED NETWORK CRYSTALS, a hypothetical class of materials, have bizarre mechanical and thermal properties: when stretched they become thicker (i.e., they have a negative Poisson's ratio) and more dense; when heated they contract. These materials, investigated by Ray Baughman of Allied Signal, Inc. (Morristown, NJ) and Douglas Galvao of the Physics Institute in Sao Paulo, Brazil, consist of twisted chains of interconnected polydiacetylene molecules. Unlike other negative-Poisson materials (generically called auxetics), these computer-designed materials derive their predicted novel properties from the interconnectivity of various parallel and non-parallel bonds in alternating layers, which allows the chains to twist, hingelike and with a minimum amount of energy expenditure, without changing the network bond lengths. Such crystals, if they could be synthesized by chemists, might also have interesting optical and electrical properties. (Nature, 21 Oct.) ATOM-OPTICS TECHNIQUES FOR MICROCIRCUIT LITHOGRAPHY keeps improving. Using the electric fields of laser light as a lens, scientists at NIST (Robert Celotta, 301-975-3710) have been able to steer a beam of chromium atoms onto a silicon substrate with great dexterity; the result is a series of thin ridges 65 nm wide, about 35 nm high, and spaced about 215 nm apart. (Science, 5 Nov.) THE PHYSICS OF BUNGEE JUMPING involves primarily the conversion of gravitational potential energy into the elastic energy of a stretched cord. Originating on Pentecost Island in the Pacific, the practice of a person jumping from a high place harnessed to a flexible attachment was introduced to Western culture in 1979 by the Oxford University Dangerous Sport Club. An all-important parameter, the amount by which the cord stretches at the bottom of the fall, should be accurately known in order to avert death. It is given by the following equation: extension = mg/K + squ root (m**2 g**2/ K**2 + 2mgl/K), where g is the gravitational acceleration, K is the cord's stiffness, L is the free length of the cord, and m is the mass of the plummeting object. (Paul Menz, Cumberland County College, in The Physics Teacher, Nov. 1993.)

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