# Create artificia gravity in space craft by using centripetal

1. Feb 21, 2015

creating artifical gravity by using centripetal force what is the formula g value

2. Feb 21, 2015

### Svein

Depends on radius and spin. Let ac be the centripetal acceleration. Then $a_{c}=\omega^{2}r$. So, if you want ac to be 9.8m/s2 and your space station has a radius of 100m - solve for the spin.

3. Jan 28, 2016

### Moderato

So what I don't get is what the spin is relative to. That's easy when you have nearby planetary systems, but when you get into deep space, any distant objects would, to all intents and purposes, have no influence on the 'gravity' being generated by the spinning of the station. So what defines the spin rate then? Surely not systems that are light years away. Does space-time itself have some fixed structure by which the spin is measured, perhaps?

4. Jan 28, 2016

### Staff: Mentor

Speeds are always relative - you have no way of knowing whether you're moving except by comparing with some external object to see if you're moving relative to it - but accelerations are not. If you were in a sealed box, no windows, no way of looking out, you'd have no way of knowing whether you were at rest or not but you'd still be able to detect (non-gravitational) accelerations using a weight on a spring or an accelerometer.

Rotating movement is accelerated movement; the centripetal force is accelerating everything towards the center. Thus, you don't need any external reference to experience it.

5. May 20, 2016

### Moderato

I understand what you say about sealed boxes, rotation and centripetal force - that is basic physics.

However, as you said - speeds are always relative - but by the same token, so is rotation. If the relative rate of rotation determines the size of the centripetal force experienced within the 'sealed box', what is that relative to? The earth, sun, solar system, centre of the universe? All of these constitute different frames of reference.

Last edited: May 20, 2016
6. May 22, 2016

### jbriggs444

Rotating reference systems are not inertial. One can determine rotation rate and direction without reference to any external bodies. One could well say that rotation is absolute, not relative.

7. May 22, 2016

### pixel

The spin is measured about the axis of rotation. We have for the tangential velocity of a point v = r ω where r is the distance to the axis and ω is the angular velocity.

8. May 22, 2016

### Staff: Mentor

Rotation is not relative.

9. May 23, 2016

### Moderato

So we can only accurately measure the rotation rate of another system by first measuring our own?

10. May 23, 2016

### Staff: Mentor

It depends on the situation and the observations that we can make. If we can measure the centripetal acceleration in the other system - maybe someone there set up an accelerometer that we can read, maybe we can watch an object dropped in the other system - then we can measure the rotation of the other system directly. The answer will come out the same no matter what our rotation rate is, so we don't need to measure ours first. (That we can measure it and get the same result no matter what ours is, is another way of saying that it is not relative).

11. May 23, 2016

### sophiecentaur

You could say that zero rotational motion is when there is no (detectable) apparent acceleration / centripetal force.

12. May 23, 2016

### Moderato

So... when there is no detectable rotation within your system, you can detect the rotation of other systems by observation, but that makes the rotation of the other system measurable relative to your non-rotating system does it not? But rotation is not relative...

Or are you saying that you can't measure the rotation of another system accurately by observation from outside of that system - the measurement has to be made from within the rotating system and then 'broadcast' to other systems? In other words visual observation of rotation will result in inaccurate measurements?

13. May 23, 2016

### A.T.

Gyros will tell you the absolute rotation of a system. The relative rotation between two systems is the difference of their absolute rotations. If you know two of these variables, you can compute the third.

14. May 23, 2016

### sophiecentaur

That seems like a logical conclusion. In principle, you could use masses on strings on three axes and go for zero tension. Another object that appears to be spinning is Really spinning. ????