# Velocity without momentum?

1. Feb 9, 2014

### bobie

In a video they state that a gyroscope precessing while rotating posesses velocity but not (angular?)momentum.

Is that true?, how can there be velocity without momentum?
Thanks

Last edited: Feb 9, 2014
2. Feb 9, 2014

### voko

That statement is wrong. A precessing gyroscope has total momentum, which is the sum of the momenta of all its "particles".

3. Feb 9, 2014

### bobie

But they put an obstacle in its way an it stopped inexplicably without resistance!!!

Last edited: Feb 9, 2014
4. Feb 9, 2014

### voko

You need to reference your source for this discussion to be meaningful.

5. Feb 9, 2014

### bobie

Here you are, voko, it's at the very end of the video

Last edited by a moderator: Sep 25, 2014
6. Feb 9, 2014

### Jilang

It's a really good video! I love the guy's delivery. I'm going to take a guess that it processes in the opposite direction to the spin. So it has energy but no net angular momentum.

7. Feb 9, 2014

### MikeGomez

If I recall correctly, Laithwaite was a pretty smart guy, but there was a problem. He was invited to the Royal Institution and he gave a lecture on gyroscopes, but the problem was that he didn't really understand gyros or why they behave as they do. It was all a horrible mess, an embarassment to the Royal Institution, and something he never really recovered from. I think that is correct, but you should look it up. Anyway, here is a sight that explains each Laithwaite experiment properly.

http://www2.eng.cam.ac.uk/~hemh/gyroscopes/laithwaite.html

8. Feb 9, 2014

### Jilang

Where is the centripetal force in #5. I'm just not getting it.

9. Feb 9, 2014

### voko

First of all, do not confuse "velocity" with "angular velocity" and "momentum" with "angular momentum". A precessing gyro, as well as a free (stable) gyro, does have angular momentum. Why it does not work as one would intuitively expect is another issue, but that is more than a mere discussion in a forum could cover. Start with a text on classical mechanics covering rigid body motion, then come back, if you still have questions.

10. Feb 9, 2014

### jartsa

I bet that if gravity disappeared suddenly, the device would continue its motion in a straight line for a long distance, until the linear motion would be stopped by air drag. So the device has linear momentum.

As the precessing stops immediately when gravity disappears, we conclude that there is no precession related angular momentum in the device. (I mean no angular momentum assosiated with the orbiting motion)

Last edited: Feb 9, 2014
11. Feb 9, 2014

### CWatters

More down here..

http://en.wikipedia.org/wiki/Eric_Laithwaite

To this day there are people who mistakenly believe you can make a reactionless propulsion system based on gyroscopes. I would not recommend using his lectures on gyroscopes to learn how they work.

12. Feb 9, 2014

### CWatters

Note that when he stops it with his hand at 7:33 the tower moves on the desk. To me this clearly shows there is angular momentum trying to keep the system rotating.

13. Feb 9, 2014

### jartsa

He grabs one end of a linearly moving hammer-like thing. In such cases there is a force felt by the grabber.