What direction would an engine spin in space? I am working on a science project in school with a friend on artificial gravity in space. We are both wondering if we could use normal electric motors, to let the outer ring move. Since there's no gravity, What direction and what part would the motor let the station spin?
 A: If the engine is to spin the ring counterclockwise with no ground holding the engine fixed, the engine itself would spin clockwise and the ring counterclockwise (actio/reactio).
A: The answer may depend on the shape of the spaceship and on how the engine is used, but there are a few general considerations that can be made.
First of all, an engine requires to be fixed to something in order to turn something else. Think for example how a car works: the engine is fixed inside the car and represents the majority of the car's mass, while the wheels are much lighter in comparison. On a ring spaceship this can be different and there is no way to avoid the formation of two different spins, one clockwise and one counterclockwise, because no part of the spaceship can be fixed. The only design I can think of is a two-ring spaceship, composed of two parallel rings side by side, spinning in opposite directions. (If it's not clear, think of it as an H, where the vertical bars are the rings and the horizontal bar is the pivot). In this way you can use an engine on each side, but there will be friction between the parts. This wear can be dangerous in space and means you'll need constant maintenance and a constant energy input or the wheels will stop.
Thrusters are a much easier solution, since there is no air resistance in space, except for the effect of the solar wind. Once the spaceship starts turning, it won't stop that easily and you'll need much less energy.
