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Suppose engineers built a cylinder-shaped space ship which rotates to create artificial gravity:

  __________________
 /<-b \             \
/      \             \  -----> a
\      /             /
 \_b->/_____________/

The space ship uses a rocket to travel through space in the direction of a. Smaller nozzles ensure that the space ship rotates in in the direction indicated by b.

  • If the space ship wants to travel in a straight line, without stopping, would the astronauts need to frequently turn on the smaller nozzles to keep the space ship rotating at the proper speed or would the space ship constantly rotate at the same speed forever?
  • If the space ship was constantly rotated counter-clockwise to create artificial gravity, would this rotation cause the space ship to drift off course?
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2 Answers 2

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There are two concepts here that may be getting mixed together, namely, conservation of linear momentum and conservation of angular momentum.

Newtons laws state that an object in motion will say in motion unless acted upon by an external force. So unless interstellar friction is a problem, the spaceship will keep travelling linearly in the same direction forever (neglecting gravitational attraction to other bodies etc).

The same applies to rotational motion. A rotating object will continue to rotate unless acted upon by an external torque. So again, unless interstellar friction is a problem, the spaceship will continue to rotate forever.

To stress again, these two things are independent - one will not affect the other.

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    $\begingroup$ A note should be made here. A body tends to have stable rotations about the minimum and maximum rotational inertia directions and unstable rotation about the intermediate axis. An unfortunate design (cough: explorer-1) would have the object change spin axes forever. Not pleasant for the passengers. $\endgroup$ Apr 19, 2018 at 20:56
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If there is no torque acting on the ship, then its rotational velocity would stay constant. The rotation of the ship has no effect on its course.

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