In space, is it possible to make a body rotate a point that does not go through its center of mass by applying external torques?

  • $\begingroup$ Yes, of course you can do this. $\endgroup$
    – john
    Jul 23 '15 at 7:50
  • $\begingroup$ Try it at home: take a straight pole/broom handle of uniform mass per unit length, and prop it up on a log/bottle lying on its side at a quarter of the distance from one end. Now push down on the short end with your hand. See how it rotates not about its centre of mass (the mid-point), but about the pivot point? $\endgroup$
    – tok3rat0r
    Jul 23 '15 at 8:36
  • $\begingroup$ (In space, the only difference is that you'd have to use your other hand in place of the log/bottle to provide the reaction force, since there isn't a conveniently placed massive object (the Earth!) to do it for you) $\endgroup$
    – tok3rat0r
    Jul 23 '15 at 8:40
  • $\begingroup$ Is the object a free object or is it anchored? What reference frame are you using, and are you including possible revolution as part of the idea of "rotation?" $\endgroup$
    – Bill N
    Jul 23 '15 at 22:32

Since a system must obey the law of momentum conservation, the center of mass of a system (which can be made of one or many bodies) must have constant velocity if no external force is applied. Hence, a body can rotate around its center of mass, or it can rotate around any other point, but only if under the influence of an external force. Therefore one can made an object rotate around a point which is not its center of mass in many ways.

For example, two object in space can rotate one around the other, i.e., around their common center of mass, under the influence of mutual gravitational attraction. This is the case of the Earth, which rotates around the common center of mass of the system which comprise the Earth and the Sun altogether.

Another possibility is to make an object rotate around a pivot which is connected to another body with a larger mass. Also in this case, the whole system (the body plus the larger body) rotates around the common center of mass.


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