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From: mikes@AMES-NAS.ARPA
Newsgroups: net.physics
Subject: Re:  cancelling forces
Message-ID: <571@sri-arpa.ARPA>
Date: Thu, 3-Oct-85 12:12:07 EDT
Article-I.D.: sri-arpa.571
Posted: Thu Oct  3 12:12:07 1985
Date-Received: Sun, 6-Oct-85 06:41:46 EDT
Lines: 36

From:  mikes@AMES-NAS.ARPA (Peter Mikes)

	Subject: cancelling forces
	----------------------------------------------------------------
	Commenting on the whole series : <10492@ucbvax.ARPA>

	>> 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. ...
	>> 
	>> -Ken Sloan
	
	No, you aren't putting as much energy into the system.  If you measured
	it, you would find that the power consumption of an electric motor drops
	when you prevent it from turning.

Actually, the useful concept, which applies equally well to two oposing robots  or two antenas, is called IMPEDANCE. When the position of two antenas is changed
so that they interfere rather then cooperate, then the impedance of the system
goes up - therefore - if rest of the system was unchanged, less (or none) energygets radiated. However, the original formulation said: " I am putting same 
energy in.." and that can be done ( e.g. the current drive is a contraption with feedback which will increase the voltage of the power supply to overcome an
 increase in impedance ) - then if it is done the dissipation will increase and the relative proportions of where the energy goes (emg, motion, heat...) depends
 on the ratio of impedances. Nice illustration of characteristic impedance is a
 long tube, closed rigidly on far end and covered with a membrane on the near
 end. Imagine you are knocking rhytmically on the membrane: When you are 'in 
 sync ' ( ressonant condition , low impedance ) the membrane is soft and gives
 easily and is accelerated by each push.  Then somebody will prolong the tube
 by half wavelenth - so that as you knock, the wave from previous push returns,
 reflected from the far end and the membrane feels hard, your knuckles ache and
 you know that impedance is high. Unless you try harder you do not accomplish as much and in any case, your energy gets dissipated and wasted...
            The moral if this nice problem is :  QUANTIFY !
         
        P.S. that 'somebody' is called fate - all it takes is 1/2 of lambda...


	


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