Why does a wheel spinning on a space ship cause the ship to rotate in the opposite direction around the ship’s center, regardless of the location of the wheel?

Question

Intuitively it makes sense when the wheel is located at the center of mass on the ship/axis of rotation, but I can’t wrap my head around how it makes sense when the wheel is located somewhere else

In: Physics

Answer 1

Newton’s Third Law, equal and opposite reactions. In order to rotate, the wheel has to apply a force to *something*. The only other something it can apply a force to is the ship. As motors turn the wheel one direction, it applies a force to the rest of the ship, which forces the rest of the ship to turn in the opposite direction with equal force.

This happens on Earth, too, if you try to turn. You have to apply a force to *something*, which is the Earth. It’s just that the force needed to spin you around when applied to the entire planet doesn’t do much of anything, so the Earth doesn’t move a noticeable amount.

It’s possible that the wheel could equal-and-opposite reaction spin itself by exhausting some gas into space. The gas goes off in one direction, and the wheel is moved in the opposite direction. This would *not* force the body of the ship to spin…except that friction still exists and the main body would tend to spin *with* the wheel instead of opposite.

There’s also [gyroscopic precession](https://www.youtube.com/watch?v=ty9QSiVC2g0) where torque is applied 90° from the rotation. But that only happens when a force is applied to the spinning object.