Dear Dr. Tom,
I just typed long reply to you. To save it in case of power cut, I right clicked mouse, select all and instead of "copy", I clicked "delete"...Where it is gone? oooops..
Any way, next time. I have sent a doc file to you, in addition to that, for the moment perhaps what I am giving below might make clear what I am saying. Please read the doc file first...
Analysis:
First Case:
Suppose initially, the point M is supporting only weight of system mg. When we first attach the body, the compression waves beginning from point A will take time to reach to point M of table. So during this propagation delay, the point M is balancing only weight mg and not (m+m')g N.
But in such situation, even if we use that weight m'g which is propagating across system and weight mg at point m of table to do work, it does not violate law of conservation in any way. There is no extra weight or force generated in this first case.
Second Case:
In our special case, we get extra force m'g generated due to propagation delay. And we get enough time to use it. Point M has no way to know that extra force is generated and it should not break.
So first case does not violate law of conservation of energy, but second case does violate law of conservation of energy.
The above situation can be explained by following anology:
Suppose you have just one billion dollar in your bank a/c and other one billion dollar deposited by you in bank is in processing and not credited to your a/c by bank. If you ask bank to stop processing and get it back to spend, you are not getting "free" money. If bank credit these one billion dollars to your a/c and you withdraw it and spend it, still, it is not "free" one billion dollars. It is hard earned money by you.
But if you have already two billion dollars in your a/c and you transfer one billion dollar from a/c A to a/c B. Now bank give credit of one billion dollar to a/c B first and debit one billion dollar to a/c A much later due to propagation delay of processing. So you have two billion dollars credit in a/c A and one billion dollar credit in a/c B. So you got three billion dollars credit to your a/cs. Now you can withdraw three billion dollars and spend that "extra free" one billion dollar.
Even if bank realise this mistake later, the damage is already done. You got it....
-Abhi.
