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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