In reply to David Roberson's message of Fri, 28 Nov 2014 23:24:51 -0500: Hi Dave, [snip] >OK, I have read several of those physics books and my position is sound. The >formula for kinetic energy as seen by an observer is E=1/2*M*V*V. That V is >not a difference, but the final relative velocity. If you want to find out >how much the kinetic energy changes you must calculate the value of kinetic >energy both before and then after the drive is applied. At that point you can >find the delta in energy and you obtain the correct value according to physics. > >How about looking at the problem from a different perspective. Let the >velocity change in two steps of 1 meter per second each. According to your >procedure the kinetic energy is 2 times the amount gained in a single step >since the same delta in velocity is determined for each one.
You are correct. My mistake was in thinking that the change in kinetic energy had to be the same for all observers. This is not so. I do spout nonsense sometimes. Thanks for keeping me on the straight and narrow. :) > >In the other case the kinetic energy is the same both before and after the >drive is enabled. The value is 1/2*1*1*M before the drive is enabled and >1/2*-1*-1*M after it completes its task. This is because the magnitude of the >relative velocity is the same both before and then after the drive finishes. >The sign changes, but that does not enter into the equation for kinetic energy >since energy is a scalar. ... but if momentum can be exchanged, then changing direction need not cost any energy. (e.g. a marble bouncing on a hard surface approaches this condition.) Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
