# How to start an artificial gravity?

I understand how artificial gravity in space stations works. It is by normal force the wall exerts on the foot.

But I wonder how to start it in the first place. I just learned about centrifugation in a centrifuge. To start, the side-wall of the tube produce a tangential acceleration. Because of the inertia (tendency to go tangentially) of the material contained, normal force is thus needed to keep the material from going through the tube and keep it rotating in a circle.

But in the space station, there is no friction, so there is no way to create that tendency that produces the need for normal force in the first place.

• Rockets mounted tangentially on the rim ? – Martin Beckett Nov 1 '15 at 19:25
• @MartinBeckett, what is that? – most venerable sir Nov 1 '15 at 19:25
• Yes, there is friction in space, not with the space, even here on Earth there is no friction with the space. And, for the last, because of the conservation of angular momentum, in space, if you put something to spin clockwise, then you would spin counterclockwise... so yeah, you can put to spin something in space. – raul Nov 1 '15 at 19:55
• I believe it's the same here. You are not considering the presence of air in your thinking. If there wasn't air, then truly things would slipped off. – raul Nov 1 '15 at 22:13
• More on artificial gravity – Qmechanic Nov 1 '15 at 22:33

You are quite correct that if you have items floating freely inside your space station they won't experience any artifical gravity as the station starts spinning. The artificial gravitational acceleration of an object is a consequence of its tangential velocity $v$ and is given by:
$$g = \frac{v^2}{r}$$
where $r$ is the distance to the axis. The freely floating objects will initially have $v = 0$ and therefore $g = 0$.