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Velocity without momentum?

by bobie
Tags: momentum, velocity
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bobie
#1
Feb9-14, 02:20 AM
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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
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voko
#2
Feb9-14, 03:15 AM
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That statement is wrong. A precessing gyroscope has total momentum, which is the sum of the momenta of all its "particles".
bobie
#3
Feb9-14, 04:49 AM
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Quote Quote by voko View Post
That statement is wrong. A precessing gyroscope has total momentum, which is the sum of the momenta of all its "particles".
But they put an obstacle in its way an it stopped inexplicably without resistance!!!

voko
#4
Feb9-14, 05:15 AM
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Velocity without momentum?

You need to reference your source for this discussion to be meaningful.
bobie
#5
Feb9-14, 06:40 AM
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Quote Quote by voko View Post
You need to reference your source for this discussion to be meaningful.
Here you are, voko, it's at the very end of the video
http://www.youtube.com/watch?v=LRYFIeU4Pcg
Jilang
#6
Feb9-14, 07:00 AM
P: 517
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.
MikeGomez
#7
Feb9-14, 08:01 AM
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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/gyro...aithwaite.html
Jilang
#8
Feb9-14, 08:11 AM
P: 517
Where is the centripetal force in #5. I'm just not getting it.
voko
#9
Feb9-14, 08:12 AM
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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.
jartsa
#10
Feb9-14, 10:22 AM
P: 476
Quote Quote by bobie View Post
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
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)
CWatters
#11
Feb9-14, 10:40 AM
P: 3,151
Quote Quote by MikeGomez View Post
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.
More down here..

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

In 1974, Laithwaite was invited by the Royal Institution to give a talk on a subject of his own choosing. He decided to lecture about gyroscopes, a subject in which he had only recently become interested. His interest had been aroused by an amateur inventor named Alex Jones, who contacted Laithwaite about a reactionless propulsion drive he (Jones) had invented. After seeing a demonstration of Jones's small prototype (a small wagon with a swinging pendulum which advanced intermittently along a table top), Laithwaite became convinced that "he had seen something impossible". In his lecture before the Royal Institution he claimed that gyroscopes weigh less when spinning and, to demonstrate this, he showed that he could lift a spinning gyroscope mounted on the end of a rod easily with one hand but could not do so when the gyroscope was not spinning. At this time, Laithwaite suggested that Newton's laws of motion could not account for the behaviour of gyroscopes and that they could be used as a means of reactionless propulsion. The members of the Royal Institution rejected his ideas and his lecture was not published.
Despite this rejection and despite the fact that Laithwaite later acknowledged that gyroscopes behave fully in accord with Newtonian mechanics, he continued to explore gyroscopic behaviour, maintaining the belief that some form of reactionless propulsion could be derived from them. Laithwaite set up Gyron Ltd with William Dawson and, in 1993, applied for a patent entitled "Propulsion System". See US5860317, GB2289757 and WO9530832 for the US, UK and pct application for patents respectively. A United States Patent, Number 5860317, was granted in 1999.
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.
CWatters
#12
Feb9-14, 10:53 AM
P: 3,151
Quote Quote by Jilang View Post
Where is the centripetal force in #5. I'm just not getting it.
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.
jartsa
#13
Feb9-14, 11:17 AM
P: 476
Quote Quote by CWatters View Post
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.
He grabs one end of a linearly moving hammer-like thing. In such cases there is a force felt by the grabber.


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