In a hypothetical world where there is no friction, and all collisions are elastic would an object in lateral motion be perpetually moving and never come to a stop?


  • Perfect vacuum: zero air friction
  • Elastic collisions: no loss of momentum
  • Zero friction with the ground
  • $\begingroup$ Conservation laws tell yes. If there is no momentum lost, it will be there forever. $\endgroup$ – Jan2103 Sep 10 '19 at 21:35
  • $\begingroup$ Elastic means no loss of kinetic energy. $\endgroup$ – my2cts Sep 10 '19 at 22:19
  • $\begingroup$ No: gravitational radiation will take energy out of the system. $\endgroup$ – JEB Sep 11 '19 at 2:22
  • $\begingroup$ How would you define moving? Relative to what. A single moving ball in a void would be indistinguishable from a stationary one. $\endgroup$ – John Meacham Sep 11 '19 at 21:12

Not necessarily. For example, if a moving ball were to hit another ball of the same mass at rest, the moving ball could stop dead, having transferred all it's momentum to the other ball.

  • $\begingroup$ but this assumes another ball was already at rest. $\endgroup$ – jmh Sep 10 '19 at 22:40

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