Does a gyroscope centripetal force?

1. Jun 10, 2006

hamilton111

After reading in specialized Physics books and different articles, I’m still thinking about gyroscopical precession. I understand everything about angular momentum, torques…, but if center of masses makes a circular movement, Where does it provide centripetal force? Could it be for the axe friction with the vertical tower? In that case, I wonder, Could it make precessing a top or a gyroscope over a no frictional floor?
Thanks a lot from Spain. (Sorry about my English)

2. Jun 10, 2006

rcgldr

I've seen gyroscopes suspended by a string from one end, and when the gyroscope's main axis is horizontal, it doesn't rotate about it's center of mass, but instead rotates approximately around the support point, with the center of mass traveling in a circle. My guess is that the string is being pulled slightly outwards by the precession of the gyroscope, and that the inwards horizontal component of tension in the string (due to gravity) counter acts this.

Need an expert here to explain this.

3. Jun 10, 2006

wywong

On a smooth surface, the gyroscope still precesses, but its centre of gravity does not travel in a circle. Instead the tips of the axis moves in circles. You can try spinning a toy top (gyroscope) on a piece of glass to see.

Wai Wong

4. Jun 12, 2006

hamilton111

I understand the examples you explain. So, my doubt is still with me. We imagine a gyroscope with horizontal precess which receives a first impulsion to avoid the nutation as the one in the picture.
It’s right for me that center of mass moves with angular and constant velocity around fulcrum. Is it right? So, it makes that it will have centripetal acceleration towards this fulcrum. Is it right? And, finally, if it’s like that and we use the equation F=Ma(center of mass) I wonder where is this force created in case that the fulcrum hasn’t got friction. Is it OK for me to apply this equation in this case? I think it would be right.
Sincerely

Alberto

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5. Jun 12, 2006

Staff: Mentor

In order for the gyroscope to precess about the fulcrum, the fulcrum must provide a centripetal component of force. If it doesn't, the gyroscope will slide off the support.

6. Jun 13, 2006

hamilton111

I agree with you, but that makes me doubt again. If no friction makes to slide the gyroscope, How does it precess the top of LEVITRON if the axe hasn’t got friction? Is it made by the friction with the air? It’s difficult for me to understand it.
Sincerely,Alberto

7. Jun 13, 2006

Staff: Mentor

I don't have one of those Levitron toys handy, but I know what you are referring to. Question: When the top precesses, does its center of mass move?

8. Jun 13, 2006

hamilton111

Yes the center of mass is jumping the whole time between two points.
Look at the video

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9. Jun 13, 2006

Staff: Mentor

That jumping around doesn't look anything like precession to me. I suspect it's due to magnetic forces acting on the top--realize that the top is a precessing magnet suspended in a non-uniform magnetic field.

10. Jun 14, 2006

hamilton111

I can say after these answers that are impossible the precession without friction and with vacuum?

11. Jun 19, 2006

rcgldr

Look at this video:

http://demolab.phys.virginia.edu/demos/pictures/1q50-23.mpg [Broken]

This system doesn't appear to rotate about the center of mass, but instead around the point of support. I suspect a small outwards angle on the string as the bicycle tire rotates around.

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12. Jun 19, 2006

rcgldr

This video answers the question: A heavy gyro suspended from a string at one end of the gyro. Rather than rotate about it's center of mass, it orbits around the support point. The orbit varies in radius, larger if the gyro is tilts downwards, smaller if the gyro tilts upwards. The explanation given is that the center of mass will maintain an altitude (I assume this would only be true if the gyro wasn't slowing down due to friction), so if the gyro tilts upwards, the orbit gets smaller because the angle of the inscribed cone becomes smaller so that the center of mass remains at the same altitude and vice versa when the gyro tilts downwards.

As previously posted by others here, the rotation of the center of mass generates an outwards force, but if the rotational rate is small, so is the outwards force. Towards the end of this video, the gyro is slowed down, and at the slower speed, it does seem to rotate about it's center of mass (or nearly so) while horizontal, then again, this isn't much different than stopping the gyro, and spinning it perpendicular to it's main axis, sort of like twirling a rope.

http://www.gyros.biz/lecture/wmv/9.wmv [Broken]

Another video, gyro suspended by string:

http://www.gyroscopes.org/video/1hi.wmv

More videos:

http://www.gyros.biz/lecture/wmv/5.wmv [Broken]

http://www.gyros.biz/lecture/wmv/7.wmv [Broken]

main web page for these videos:

http://www.gyroscopes.org/1974lecture.asp

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13. Jun 19, 2006

rcgldr

Doc Al is correct. If you look at a typical gyroscope, the end points are shaped like balls, and the tower tops are cup like to provide the small amount of centripal force to keep the gyro from sliding off.

As seen in the case of the gyros suspended by string, there is an outwards force due to the rotation of the center of mass.

14. Jun 20, 2006

hamilton111

Thanks a lot for the videos.Unfortunately my english is not so good and I cannot understand whet they say.
It´s right for me that the center of mass rotation makes a centrifugal force. All of you say that it´s very small,but is it possible to be calculated through mv2/r or is it smaller that mv2/r ?
On video 7.wmv how can it be gyroscope not falling?
Why isn´t the force of gravity "mg" transmited by the support point to the floor?
It´s surprising for me.
Sincerely
Alberto

15. Jun 20, 2006

rcgldr

It's mv^2 / r, but v is very low in these cases.

Because the gyroscope is creating a torque force, in the direction that is upwards on the non supported point, and downwards on the supported point. The net result is there is a downwards force that equal (on average) to the weight of the gyroscope at the support point, not at the gyroscopes center of mass, so the platform doesn't fall.

Here's another example of this princple:

http://www.gyros.biz/lecture/wmv/8.wmv [Broken]

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16. Jun 25, 2006

hamilton111

I don’t understand quite well a question. If the platform doesn’t fall is because the force on the top of the tower is very small, isn’t it? Much smaller than the weight of the gyroscope. If the tower makes another equal reaction upwards, is it right that the F=ma(cm)? If the gyroscope is under the action of weight (downwards) and a small reaction (upwards), the center of mass should fall or not? Could you send me a forces’diagram?
What I watch in videos 7, 8, does it happen with a simple toy gyroscope (non articulated)? I think in this case the gyroscope would make the platform fall.
Sincerely. Alberto.