From: heathjohn2@gmail.com   
      
   On Wednesday, March 21, 2018 at 8:00:05 AM UTC-4, Edward Prochak wrote:   
   > On Friday, March 9, 2018 at 2:59:51 AM UTC-5, John Heath wrote:   
   > > On Wednesday, March 7, 2018 at 1:35:44 PM UTC-5, Edward Prochak wrote:   
   > > >   
   > > > The solution is simple as long as the rules are applied correctly.   
   > > > Words fail to reach the correct answer in your example.   
   > > >   
   > > > At start net angular momentum is zero L = I * w = 0   
   > > > (Using w for omega, the angular speed)   
   >   
   > > > So looking at just the person the angular momentum is initially   
   > > >   Lp = Ip * wp   
   > > >   
   > > > Now holding the elastic band, you stretch your arms.   
   > > > The momentum is conserved Lp = Ip' * wp'   
   > > > but the rotational speed changed because of the change in the   
   > > > moment of inertia (Ip).   
   > > >   
   > > > Now you lock the elastic band. You return your arms to their original   
   > > > position, so we get to Lp = Ip" * wp"   
   > > > Note this is double prime values, BECAUSE while the elastic band has   
   > > > very little mass, it does contribute to the moment of inertia I.   
   > > > So the final moment of inertia Ip" =/= Ip' and the rotational   
   > > > speed is proportionally different also.   
   > > >   
   > > > So you conclusion is wrong. The momentum of the box + person   
   > > > started at zero and remained at zero the entire process.   
   > []   
   > > You are over thinking. There is a short cut. You did not stretch the   
   > > elastic band. Only the tendency of arms to spread out while spinning   
   > > stretch the elastic band. Only energy from angular momentum stretched   
   > > the elastic band not human power. With this in mind your short cut is   
   > > energy was gained to stretch an elastic band. It did not come from the   
   > > human energy so it had to come from angular momentum. As the the human   
   > > was on a turn table when these actions took place the angular momentum   
   > > of the box could not be effected. Therein is the rub. The equal angular   
   > > momentum of the box and human on the turn table are no longer   
   > > equal. Angular moment as not been conserved. The one on the turn table   
   > > gave part of his angular momentum to stretch an elastic band. In effect   
   > > nature was taken momentum in exchange for energy , stretched elastic   
   > > band. As long as the elastic band remains stretched ENERGY conservation   
   > > laws prevent restoration of angular momentum conservation between the   
   > > box and the human on the turn table.   
   >   
   > I did not specify the source of energy to stretch the band.   
   > it could have been simply as you said.   
   >   
   > The problem is that you are confusing angular speed and   
   > angular momentum.  The angular speed is indeed different   
   > after stretching the band, but the angular momentum of the   
   > combined Person, elastic band, and turntable system remains   
   > the same. This is because the moment of inertia changed also.   
   >   
   > Linear inertia is due only to the total mass of the object.   
   > The moment of inertia is dependent on the total mass and the   
   > distribution of that mass (IOW, its shape).   
   >   
   > So there is no rub which allows you to escape. It may be time   
   > for you to do a little experimenting. Note that since you are   
   > only dealing with the person, band and moving part of the   
   > turntable, you can do the experiment at home.   
   >   
   > But again I suggest that the "band" be something massive that   
   > can make a more significant change in the moment of inertia.   
   >   
   > It doesn't have to be elastic like a spring. A heavy weight   
   > tied to two handles via pulleys. As you hold the handles   
   > and widen your arms, it lifts the weight which is at the   
   > center of the rotation. Make the handles something also heavy   
   > (heavy enough to change the moment of inertia, but light   
   > enough that even rotating, they can lift the heavy weight).   
   >   
   > But also, you really need to look at the equations and work   
   > a few examples, to see that there is no free energy here.   
   >   
   > Enjoy,   
   >   ed   
      
   You have made some good point and I agree. From this let us establish   
   a foundation that can be trusted.   
      
   A]There is no free energy. All forms of energy must be accounted   
   for.   
      
   B] Linear momentum conservation in x y and z direction can not be   
   violated.   
      
   C] Angular momentum conservation may be violated by exchanging it   
   for energy , stretch elastic band.   
      
   You agree with A and B but you are not liking C. If you have a   
   stretched elastic band in your pocket caused by angular momentum   
   and not willing to reduce angular momentum you in violation of A.   
   Do you have an alternative source of energy that stretched the   
   elastic band?   
      
   [[Mod. note -- Your statement (C) is false.  Angular momentum   
   is conserved, and there is no way to violate that conservation.   
   As other people have pointed out, your stretched elastic band is   
   not violating conservation of momentum.   
      
   Your stretched elastic band is also not violating conservation of   
   energy.   
      
   It might be instructive to consider a different example of stretching   
   an elastic band.  Let's consider a room on the Earth's surface,   
   containing a heavy weight which is clamped in place, and an   
   unstretched elastic band connecting the weight to the ceiling of the   
   room.  Now move the tabletop out from under the weight, so that the   
   weight falls down to the floor, stretching the elastic band as it   
   does so.  For simplicity, let's assume that the elastic band stretches   
   sufficiently to slow the weight's fall such that the weight comes to   
   a stop (zero instantaneous vertical velocity) before the weight hits   
   the floor, and that at that moment we then clamp the weight again.   
      
   So... we now have a stretched elastic band.  Where did the energy   
   come from to do that stretching?  Clearly it came from the gravitational   
   potential energy of the weight, i.e., it came from the Earth's   
   gravitational field doing work on the weight as the weight fell down   
   towards the Earth's center.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)