Carl Mungan

• Harvey Leff has begun a five-part series of articles on demystifying entropy on page 28 of the January 2012 issue of The Physics Teacher. In addition, two articles particularly caught my eye in the December 2011 issue. Craig Bohren discusses convective and radiative cooling in "Why do objects cool more rapidly in water than in still air?" He points out that a person can survive a long time in still air at 7°C but not in water at the same temperature. Secondly, on page 567, an experiment is performed to measure the slipping angle of a ladder leaning against a wall when there is friction at both the floor and wall, unlike the usual textbook case that assumes a smooth wall. I have been interested in this same problem (see http://usna.edu/Users/physics/mungan/Scholarship/RotationalEquilibrium.pdf), but it is the clever technique used here to keep the two ends of the ladder perpendicular to the floor and wall that validates the theory.
  
  
• Under the editorship of David Jackson, the American Journal of Physics is continuing to publish a diverse range of interesting problems. Check out Behroozi's article in the Nov. 2011 issue experimentally relating the internal pressure of soap bubbles to their radius and surface tension. In the Dec. issue, learn about video measurements of the added mass of a thrown beach ball, arising from the fact that the moving ball must accelerate some air around it. There is also a fabulous brain twister involving a falling block that is connected by a string and pulley to a second block sliding on a frictionless horizontal track. On page 24 of the Jan. 2012 issue are measurements of the muzzle velocity of a compressed air cannon; it turns out that primary disagreement with simple theory is not because of friction as the ball moves down the barrel, but because of the pressure drop as air flows through the valve between the pressure tank and the gun. Finally, I found Corti's note on the Gibbs paradox in the Feb. issue to be particularly enlightening in understanding the tricky issue of distinguishability of ideal gas particles.
  
  
• Most of us are probably familiar with the idea that one can tell the Earth is round because the mast of a ship progressively sinks below the horizon as it sails straight away from shore. Kibble puts some numbers on a photo of a distant bridge to calculate Earth's radius on page 685 of the November 2011 issue of Physics Education. The journal can be accessed at http://iopscience.iop.org/journals.
  
  
• The same webpage also gives a link to the European Journal of Physics. The November 2011 issue has an article by David Rowland and three letters by Butikov, by Burko, and by Repetto et al. discussing the surprisingly complicated matter of the correct expression for the potential energy density of a transverse wave on a string. The issue is that one needs to take correct account of longitudinal motion of string segments, which must occur if the stretching is uniform along the string. I have collected together some related discussions from the past decade of the momentum carried by mechanical waves at http://usna.edu/Users/physics/mungan/Scholarship/WaveMomentum.html.
  
  
• The November 2011 issue of Journal of Chemical Education has an article by Fieberg and Girard suggesting a pie mnemonic for relating the various thermodynamic potentials such as enthalpy and Helmholtz free energy, along with the corresponding Maxwell relations.
  
  
• The Fall 2011 issue of the Center for Excellence in Education Newsletters interviews a number of physicists about the issue of Physics First in high schools.

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