# Thread: Physics department: Mach's principle (no, not airspeed)

1. ## Physics department: Mach's principle (no, not airspeed)

There's too little physics talk going on in CPF so I figured I throw this one in and see what happens

From wiki:

You are standing in a field looking at the stars. Your arms are resting freely at your side, and you see that the distant stars are not moving. Now start spinning. The stars are whirling around you and your arms are pulled away from your body. Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move?

Mmm.. theoretical physics..
Are there any physicists around here? What happens when you look at the stars, start spinning, get dizzy and fall? Is it the universe striking back?

2. ## Re: Physics department: Mach's principle (no, not airspeed)

In other words - centripetal force is based on rotation speed, but...rotation relative to what? For that matter, all fast objects have their momentum greatly changed as you increase velocity towards that of light - but velocity relative to what?

"It's all relative, but at least there's beer."

3. ## Re: Physics department: Mach's principle (no, not airspeed)

circular acceleration in reference to your previous state. Complicated in that it shouldn't matter what your previous state was? I dunno, I think it depends on how drunk you are.

4. ## Re: Physics department: Mach's principle (no, not airspeed)

Definitely concur with the above.

Edit:
And here's one that makes me think:

Why do moving objects such as fan blades accumulate more dust on them?

5. ## Re: Physics department: Mach's principle (no, not airspeed)

Originally Posted by Kestrel
Why do moving objects such as fan blades accumulate more dust on them?
It turns out that the fan blade acts as an electrostatic filter, collecting opposite-charged particles. These tiny particles do not bounce on impact - they stick! You can get some FANTASTIC dendrite structures this way.

6. ## Re: Physics department: Mach's principle (no, not airspeed)

+

Angular Momentum
is much more interesting

________________^ Katrina __________________________^ Pinwheel galaxy via Hubble

7. I'm schooled in biology, so I'll comment on the dizziness question. Had to take two semesters of physics anyways though.

You get dizzy after spinning because throughout that motion you get the fluid in the semicircular canals in your inner ear sloshing about and bending tiny tiny hairs in there. The movements of those hairs tells your brain your head's position. When you stop spinning suddenly the fluid keeps sloshing for a few moments, bending hairs and making your brain think you are moving even though you have stopped.

8. ## Re: Physics department: Mach's principle (no, not airspeed)

Angular Momentum is much more interesting
Awesome picture comparison BTW. Things like this really make you think.

A big difference between the hurricane and galaxy is the stars in theouter arms of the galaxy are moving at nearly the same velocity as the inner stars. Something that has baffled physicists for quite some decades.

9. ## Re: Physics department: Mach's principle (no, not airspeed)

In other words - centripetal force is based on rotation speed, but...rotation relative to what?
I would not see it as rotation, but constant acceleration. If someting moves in a circular motion, it constantly changes direction. To make something change direction, you have to accelerate it in the new direction. To accelerate something, you need to have a force act on it. Every force has an equal force, working in the opposite direction. The latter is what you feel in your arms.

When you stop spinning suddenly the fluid keeps sloshing for a few moments, bending hairs and making your brain think you are moving even though you have stopped.
Correct.
When the sensory information of different senses do not match, sichness can happen.
I always got sich in a car as a kid, when I was sitting relatively still and my eyes detected modion from the moving ladscape.

10. ## Re: Physics department: Mach's principle (no, not airspeed)

Thanks guys for interesting thoughts and nice pics!
Apparantly, Mach's principle even caused Einstein some headache so it's not simple.

11. ## Re: Physics department: Mach's principle (no, not airspeed)

If we can measure rotation, then there is some amount of rotation that is 0. But 0 relative to what? A magic 'ultimate reference frame,' or just to the average position of all masses in the universe? Could one manipulate rotation frames with very large, dense shells?

12. ## Re: Physics department: Mach's principle (no, not airspeed)

I posted this thought experiment in a physics forum some time ago (yes, I'm a nerd):

Imagine you have two spheres floating freely in space. One, "A", is rotating at high speed around its axis, the other, "B" is in rest. The rotating sphere bulges due to the centripetal acceleration, the other sphere does not.

Now, remove the rest of the universe but remain as a passive, invisible observer

Am I correct in assuming there is no way for nature to know if it is "A" that is rotating around its axis, or if "B" is cirulating "A" like a satelite, always having the same face directed towards "A"?

And if so, why is "A" bulging while "B" is not? If "B" is circulating "A", it would be rotating around its axis with the same angular velocity, so should it not also bulge?

13. ## Re: Physics department: Mach's principle (no, not airspeed)

Originally Posted by baxtrom
I posted this thought experiment in a physics forum some time ago (yes, I'm a nerd):

Imagine you have two spheres floating freely in space. One, "A", is rotating at high speed around its axis, the other, "B" is in rest. The rotating sphere bulges due to the centripetal acceleration, the other sphere does not.

Now, remove the rest of the universe but remain as a passive, invisible observer

Am I correct in assuming there is no way for nature to know if it is "A" that is rotating around its axis, or if "B" is cirulating "A" like a satelite, always having the same face directed towards "A"?

And if so, why is "A" bulging while "B" is not? If "B" is circulating "A", it would be rotating around its axis with the same angular velocity, so should it not also bulge?

That's the trouble here. The test is easy: Spread jelly on each sphere. One that is rapidly rotating will fling the jelly off about its equator. One that is orbiting will fling it from one side. One that is neither orbiting, nor rapidly revolving, will remain deliciously coated. But...why is this? What is rotation 'relative' to that we can perform such a simple test?

14. ## Re: Physics department: Mach's principle (no, not airspeed)

Originally Posted by Kestrel
Definitely concur with the above.

Edit:
And here's one that makes me think:

Why do moving objects such as fan blades accumulate more dust on them?

I'd have to guess that they pick up a static electrical charge maybe, and they do come in contact with more dust than just about every other object in a room. Thats because they are sucking dust through them, and that dust is floating in the air, or is picked up from the floor, or from the surfaces of other objects and sucked through.

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