# What is the gravity inside a rotating cylinder?

There is the popular question “What is the gravity at the center of the Earth?”. And the answer is zero, because the forces cancel out. And then the gravity increases linearly as you move to the surface. Would the same be true for a rotating cylinder?

I mean, if, for example, I have a cylinder with a radius 270 of meters, rotating at 2 rpm, you can calculate, that the gravity will be about 1.21 g on the surface of the cylinder, right? And then, if I am inside and I go deeper (towards the center) in the cylinder while it is still spinning, would the gravitational force decrease to zero (for example, when I'm 80 meters in, so in about 190 meters distance from the center, would be in a place with 0.8 g)?

• @Hobbes says it but I'llm add it here again for anyone who might miss it - this is not real gravity, it is (roughly speaking) the floor accelerating up towards you. So there is never any actual gravitational force due to the rotation, and the artificial gravity "disappears" as soon as you stop spinning around the axis.This is an interesting question! – uhoh Jan 7 '17 at 20:19
• @BenCrowell That applies to linear acceleration, but I don't think that applies to the centrifugal force created by rotation used as a source of artificial gravity. For example, the biggest problem with rotation-derived artificial gravity is the Coriolis force which would make it feel very clearly artificial, uncomfortable and hard to move around. You would know instantly you were in a rotating frame, without question. You can read the several good answers here, and this concise answer as well. – uhoh Jan 8 '17 at 6:29
• @BenCrowell here is a nice illustration of somebody "discovering" their local surroundings are a rotating frame. – uhoh Jan 8 '17 at 7:08
• @uhoh: The basic machinery of general relativity doesn't make any distinction between linear and nonlinear acceleration. The technical statement of this fact would be that field equations are invariant under any smooth change of coordinates. Transforming to a rotating frame is just one type of change of coordinates. The equivalence of a gravitational field to the choice of a noninertial frame is in general only a local equivalence. In your examples, the observer needs to explore a nonvanishing volume. Note that we are not disagreeing. I'm saying that all gravity is fictitious. You're [...] – Ben Crowell Jan 8 '17 at 17:34
• [...] saying that not all fictitious forces can be interpreted as a uniform gravitational field over a large region of space. These are not logically in conflict. – Ben Crowell Jan 8 '17 at 17:35