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# Ring on a string spinning horizontally... watch

1. Hi,

How would you resolve vertically in this case?

Say you have a piece of string and you slip a ring through to the middle, and then you spin the ring horizontally.

Would it be:

But then mg = 0...which is wrong right?

Would the top and bottom angles be symmetrical?

Or can you only resolve horizontally in this case?

Thanks
2. Well are you assuming it is spinning perfectly horizontally or do you know the angle it makes to the vertical?
3. (Original post by Rubgish)
Well are you assuming it is spinning perfectly horizontally or do you know the angle it makes to the vertical?
What do you mean spinning perfectly horizontally? Yes, you I am saying the angle it makes with the vertical is theta.

Ill draw a diagram if you dont understand...
4. Yeah mg cannot be 0 cause then 9.8m = 0 and m=0 in which case there is no particle :P

Assuming the ring is smooth (so that both tensions are the same, if not use T1 and T2).

The top and bottom angles are not the same otherwise you will get m=0 as you saw before. You need to have 2 different angles there. (where theta and alpha are angles between the vertical through which the particles weight acts and the string).
5. Yes the ring is smooth kinda like this:

So you are saying, that if I were to experiment, I would never ever be able to get theta and alpha to be the same degree? (assuming that the ring is in equilibrium)
6. Yes you would be able to because forces like air resistance and such exist in practice so mg wouldn't equal 0, it would be affected by those forces. Also, good luck spinning it in a perfectly horizontal plane :P

It won't happen on an exam paper anyway.
7. I think it's likely that if you did an experiment you would be able to get alpha and theta equal as friction between the ring and the string would balance the gravitational force provided you spin it fast enough.
8. (Original post by Tla)
Yes you would be able to because forces like air resistance and such exist in practice so mg wouldn't equal 0, it would be affected by those forces. Also, good luck spinning it in a perfectly horizontal plane :P

It won't happen on an exam paper anyway.
But say the two ends of the string are directly above and below eachother (like its meant to be)....and you spin the ring fast enough so that it sits in equilibrium in exactly the middle of the string, wouldn't alpha and theta be the same? They would have to be the same right? (not including air resistance and stuff here)

Sorry, it's just that Im a bit skeptical, as I would have imagined that it would be the same angle >__>
9. (Original post by 2710)
But say the two ends of the string are directly above and below eachother (like its meant to be)....and you spin the ring fast enough so that it sits in equilibrium in exactly the middle of the string, wouldn't alpha and theta be the same? They would have to be the same right? (not including air resistance and stuff here)

Sorry, it's just that Im a bit skeptical, as I would have imagined that it would be the same angle >__>
They would have to be the same yes. But you've already seen why it's not possible.
10. (Original post by Tla)
They would have to be the same yes. But you've already seen why it's not possible.
So what's not possible about my situation though? Say you did the experiment in a vacuum and it was spun by a machine, and you have managed to acquire a frictionless ring from another galaxy. Just because the model says it is not possible, why wouldn't it work in real life, using this scenario:

(Original post by 2710)
But say the two ends of the string are directly above and below eachother (like its meant to be)....and you spin the ring fast enough so that it sits in equilibrium in exactly the middle of the string, wouldn't alpha and theta be the same? They would have to be the same right? (not including air resistance and stuff here)
11. Well in real life it's technically impossible to get the ends of the string directly above and below each other and the ring spinning in the exact middle of the string, but that's physics not maths :P

But if you did, and everything else was EXACTLY like the model then it would be impossible.
12. (Original post by Tla)
Well in real life it's technically impossible to get the ends of the string directly above and below each other and the ring spinning in the exact middle of the string, but that's physics not maths :P

But if you did, and everything else was EXACTLY like the model then it would be impossible.
Ugh...>__>

I don't think you can just say it is impossible. I was trying to represent my model in a mechanical world where everything is perfect. I was looking a for an answer along the lines of: Well, that would never happen because the ring would move to position itself so that the tensions balance. Ie. Equilibrium at the middle is not possible. Something like that which would explain it. At the moment, I still do not understand ¬_¬. Well, I guess I'm just overthinking

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