Mechanical energy swing because of the work done by the pendulum to oppose these non-conservative forces. That the loss of mechanical energy in a system always resulted in an increase of the system's temperature has been known for a long time, but it was the amateur physicist James Prescott Joule who first experimentally demonstrated how a certain amount of work done against friction resulted in a definite quantity of heat which should be conceived as the random motions of the particles that comprise matter. This equivalence between mechanical energy and heat is especially important when considering colliding objects. In an elastic collision,
Feathers: The Evolution of a Natural Miracle and how feather insulation has also been utilized by humans. "Flight" discusses how flight might have evolved in birds, interviewing Prum, Feduccia, Xu, and also ornithologist Ken Dial, who described wing-assisted incline running. In "Fancy", Hanson discusses the role of feathers in sexual selection, as well as how humans have utilized feathers for fashion, interviewing costume and fashion designers on the Las Vegas Strip and New York City. "Function" discusses how feathers have been adapted for other purposes, such as waterproofing, fly fishing, and quill pens. Hanson decided he would write "Feathers" after going on a run and noticing a