Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site oddjob.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxn!ihnp4!gargoyle!oddjob!matt From: matt@oddjob.UUCP (Matt Crawford) Newsgroups: net.physics Subject: Re: Potential Energy (could someone expand on the 'yes' answer?) Message-ID: <1035@oddjob.UUCP> Date: Tue, 5-Nov-85 13:03:21 EST Article-I.D.: oddjob.1035 Posted: Tue Nov 5 13:03:21 1985 Date-Received: Thu, 7-Nov-85 04:42:34 EST References: <175@tulane.UUCP> <471@iham1.UUCP> <536@talcott.UUCP> <1514@teddy.UUCP> <1076@jhunix.UUCP> <1235@mhuxt.UUCP> Reply-To: matt@oddjob.UUCP (Matt Crawford) Organization: U. Chicago, Astronomy & Astrophysics Lines: 30 Summary: The following digresses somewhat from the original question, but it is all relevant. The predominant meaning of the word "mass" among physicists is "rest mass", not mass times some velocity-dependent factor. For a single particle with energy E and momentum P (consider all P's to be vectors in this article), the mass is the square root of E^2 - P^2. This does not depend on the frame in which E and P are measured. Often physicists will talk about the mass of a pair of particles. this is just the square root of (E1 + E2)^2 - (P1 + P2)^2. If the two particle are the sole decay products of some other particle, then this is the mass of that original particle. (Side note: the combined mass of the decay products will be distributed about the mean with a standard deviation inversely proportional to the half-life of the particle that decayed to produce them. Thus one hears phrases like "the width of the Z-0".) If you compress a spring with a rock at each end, its mass will be greater than the separate masses of the rocks and uncompressed spring. The extra mass will equal the kinetic energy which the spring can give to the rocks (divided by c^2, if you like). If you separately square the total energy and total momentum of the spring and rocks, then subtract, you will get the same answer both while the system is tied together and after the rocks are sent flying. _____________________________________________________ Matt University crawford@anl-mcs.arpa Crawford of Chicago ihnp4!oddjob!matt