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

 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.)