Orbiting cylinderworld

Classical Mechanics Level 3

In your interstellar voyages, you have discovered a strange new planet! Instead of the traditional geoid shape, this planet is a nearly infinite cylinder!

In order to safely enter orbit, you need to understand the nature of orbital mechanics near this strange planet. If you are in a circular orbit around this planet, and you wish to enter a larger circular orbit, will your speed in the larger circular orbit be higher, lower, or the same as your speed in the smaller circular orbit?


  • The question asks about the speed in a larger orbit relative to the speed in a smaller orbit - not the speed needed to change orbits.
  • You should treat this situation classically with Newtonian gravity; no special or general relativity.
  • You may assume the planet's mass is distributed uniformly along the length of the cylinder
  • You may assume that the length of the cylinder is effectively infinite (much larger than the radius of the orbits).
  • You may assume other complicating factors like atmospheric drag or the dimensions of the spacecraft itself are negligibly small.

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