From: niklas.holsti@tidorum.invalid   
      
   On 2021-12-23 23:32, JF Mezei wrote:   
   > On 2021-12-23 06:49, Niklas Holsti wrote:   
   >   
   >> Eventually it will, but initially the acceleration propagates at the   
   >> speed of sound in the pole. The far end does not move until the   
   >> compression wave reaches that end.   
   >   
   > Thanks for explanation.   
   >   
   > Is the compression happening at the atomic level, or at the material   
   > level (like a sponge/spring)?   
      
      
   Could be either or both, depending on the structure of the pole. If the   
   pole is compact steel, for example, the compression is at the atomic   
   level, but if you build a series of disks and springs as I described, it   
   would be at both levels, with most of the compression (measured by   
   change in length) in the springs.   
      
      
   > So when I push that 147 million km long pole, does science know how much   
   > mass I will feel pushing back?   
      
      
   If by "mass .. pushing back" you mean how much a given push force will   
   accelerate the pushed end of the pole, you have to consider separately   
   the dynamic case (when the force is first applied and shortly   
   thereafter) and the static case (constant force for a long time).   
      
      
   > I assume at that scale, I won't see any difference whether the pole   
   > is 147 million km long or 300 million km long?   
      
   In the static case (constant push for a long time) you will certainly   
   feel the full 300 million km of pole, and it will feel over twice as   
   massive as the 147 million km pole.   
      
   In the dynamic case, if you suddenly apply a push to one end of the   
   pole, the first acceleration will occur just at the surface where the   
   push is applied, thus it will feel like a very small mass. But that will   
   last a very short time because the push will quickly propagate along the   
   pole, and as quickly the acceleration will decrease as more and more of   
   the mass of the pole is involved.   
      
   Comparing the 147 million km pole and the 300 million km pole, you   
   should not see any difference until the compression wave has reached the   
   147 million km point. The situation after that becomes more complex as   
   the wave in the shorter pole is reflected back from its far end, while   
   the wave in the longer pole continues to propagate.   
      
      
   > If I impart 1 Newton at one end, I take it I get an immediate 1 newton   
   > "equal reaction"   
      
      
   Of course. That is more or less how "force" is defined... However, if   
   you want to measure the applied force with some kind of dynamometer, you   
   can do that easily in the static case, but in the dynamic case you would   
   have to include the compressibility and sound speed of the dynamometer   
   itself as corrections to the measurement (in fact, you must consider the   
   whole dynamic frequency response function of the dynamometer).   
      
      
   > and the pole will figure out the push propagates within   
   > itself?   
      
      
   Yes. Poles are very intelligent and can certainly figure out such things.   
      
      
   > (I assume that if I impart 1 Newton onto the space station, I get the   
   > same reaction against me as uf I imparted 1 newton against that 147   
   > million km long pole?   
      
      
   Theoretically yes. But in the dynamic case, any real "piston" or other   
   mechanism that you could use to apply the push would not be able to   
   maintain a constant 1 Newton force when the compression waves (and   
   flexures, for the space station) in the pushed object return to the push   
   point and accelerate it (move it about). If that acceleration is towards   
   the pushing mechanism, the force will increase; if it is away from the   
   pushing mechanism, the force will decrease; before the mechanism can   
   react and restore the 1 Newton force (until the next wave comes in). The   
   pushing mechanism would be able to sense these accelerations and force   
   disturbances quickly in the space station case, because the station is   
   small, while for the long pole they would happen much later.   
      
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