Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/3/84; site teddy.UUCP Path: utzoo!linus!philabs!cmcl2!harvard!talcott!panda!teddy!rdp From: rdp@teddy.UUCP Newsgroups: net.physics Subject: Re: cancelling forces Message-ID: <1325@teddy.UUCP> Date: Fri, 20-Sep-85 09:48:27 EDT Article-I.D.: teddy.1325 Posted: Fri Sep 20 09:48:27 1985 Date-Received: Sun, 22-Sep-85 16:31:36 EDT References: <546@sri-arpa.ARPA> Reply-To: rdp@teddy.UUCP (Richard D. Pierce) Organization: GenRad, Inc., Concord, Mass. Lines: 52 In article <546@sri-arpa.ARPA> sloan@uw-tanga.arpa writes: >From: Kenneth Sloan > >Let's say I have a robot that pushes a box. I put a certain amount of >energy into it, and get most of that energy out as work performed on >the box (the rest being lost to maintain the robot's life support systems). > >Now I set up another one of these, and place it alongside the first >robot. I have them push in the same direction so that the forces add. >Now the output of this system is a moving box with the same direction >and twice the speed. I'm putting twice as much energy in and getting >twice as much energy out. > >Here's the question... If I place them on opposite sides of the box, >the pushes will cancel. Now I appear to be getting no energy out of >this system, at least not in the form of a moving box. I am still >putting as much energy into the system. Wrong, you are putting no "energy" into the system, : 1 2 Energy = - * Mass * Velocity 2 Since the net velocity is 0, then the total energy is also 0. Note also that no work is done (save the robots grinding clutches and the like), since work is a measure of the rate of enrgy expenditure. NOw, this is not to say that no power will be drawn from the robots power sources. As alluded to above, these poor little suckers are going to be weeping, wailing and gnashing gears trying to go nowhere, generating all sorts of heat, etc. Let's re-do your experiment a bit to help illustrate. Put our box on a frictionless table. now attach a line and let it pass over the edge of the table, where it is attached to a weight. Sure enough, the box will move (more accurately accelerate) under the influence of the force exerted by pulling on the weight. What has happened here is the potential energy of the weight is being converted into kinetic energy of motion. NOw, on the opposite side of the box, attach another line with a weight of identical mass, hanging over the edge of the table. Voila! the box does not move. The potential energy of the ssystem does not change because it is not being converted into kinetic energy of motion. To argue that both weights are exerting "energy" violates the conservation law of energy, since this energy is would be irretrievably lost. What is happening, very simply, is that there is a perfect balance of forces, resulting in no net force, causing no net motion, therefore no net energy use. Dick Pierce Brought to you by Super Global Mega Corp .com