Look at this:


The chain seems to be moving with a uniform velocity. Is the kinetic energy of all the moving beads in the whole chain (at the time they reach the ground) the same as the kinetic energy (when touching the ground) of the whole chain when you let it fall freely from the same height as the jar? Or move the beads in the chain with the velocity a single bead would have (when reaching the ground) as you let it fall from jar height? In both cases, the chain possesses the same potential energy.


closed as unclear what you're asking by Kyle Oman, sammy gerbil, Jon Custer, John Rennie, rob Jan 20 '17 at 19:36

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The bead chain on the outside falls down rather quickly, and since the beads are connected, the beads on the inside of the beaker must be traveling upward just as quickly. When they get to the edge of the beaker, however, they want to start traveling downward instead. But they can't just change directions: they have momentum upwards, and it takes time for the force of gravity to redirect their momentum downwards. In this time, they travel some distance upwards, thus creating the loop that you see.

  • $\begingroup$ I understand the beads can't change direction at the edge of the jar (it would require infinite force, dp/dt). But if you add up the kinetic energies of all the beads in the chain when they touch the ground, is it the same as the kinetic energy of all the beads in the chain if you let the chain fall as a whole fall freely from the height of the jar. In both cases, the kinetic energy has to be the same because the potential energy of the chain is the same in both cases. Is it because the beads in the chain fall from a higher height as the whole chain does? $\endgroup$ – descheleschilder Jan 20 '17 at 9:01
  • $\begingroup$ @descheleschilder Yes, the average kinetic energy of each bead has to be the same as if you just drop the chain. The beads fall from a larger height, but there's an upwards force that slows them down, so they aren't going any faster when they hit the ground then if you just dropped them. $\endgroup$ – Chris Jan 20 '17 at 18:55

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