# Thread: Plz hep,l i dont see how angular momentum is conserved in this case

1. All right so an astronaut has a belt with two counter rotating gyroscopes which net momentum is zero

But he has a fixed point in space to make a satellite spin as much as he wants

So do you see where I am confused here?

initial momemtum=zero

final momentum=certain amount(the astronaut imput in the satellite)

2.

3. You need to better describe what is going on. However if you set something in motion, something else will have the opposite momentum.

4. Originally Posted by luxtpm
But he has a fixed point in space to make a satellite spin as much as he wants
How exactly would he do that ?

5. What characteristics do you think two counter-rotating gyroscopes have?

6. I dont know, take it the giroscopes are heavy and very fast

Taking into account they are counter rotating its momemtum will be zero...

everybody knows a giroscope offers resistance to be spinned

in fact i think the astronauts use this to use a screw driver in free float

7. Originally Posted by luxtpm
everybody knows a giroscope offers resistance to be spinned
That is not actually correct. The orientation of the spinning disk inside the gyroscope tends to remain the same regardless of how the gyroscope is mounted, but no actual resistance ( as in : force ) is offered. Thus I am not sure how your idea with the satellite would work...?

8. Originally Posted by Markus Hanke
Originally Posted by luxtpm
everybody knows a giroscope offers resistance to be spinned
That is not actually correct. The orientation of the spinning disk inside the gyroscope tends to remain the same regardless of how the gyroscope is mounted, but no actual resistance ( as in : force ) is offered. Thus I am not sure how your idea with the satellite would work...?
Yes that is what i meant i did not express well

the point is the astronaut has now a "fixed" point to use a crewdriver

9. Originally Posted by luxtpm
the point is the astronaut has now a "fixed" point to use a crewdriver
No he doesn't - what happens is that, while the orientation of the disk inside the gyroscope remains the same, the outside housing of the instrument rotates freely, therefore not offering any resistance. If you were to try and attach yourself to the spinning disk inside, than the disk would no longer maintain its orientation - it also does not offer any resistance. Bearing that in mind, the gyroscope does not offer any "fixed" point for the austronaut, and so the angular momentum is of course preserved.

10. Originally Posted by luxtpm
in fact i think the astronauts use this to use a screw driver in free float
I don't think astronauts would use a screwdriver ( or any other tool ) in free float, particularly if those are power tools. I would think they would attach themselves to something first. I am not sure though - anyone got any info on this ??

11. I recall that there were a whole set of tools and processes developed to deal with this problem, but I don't recall the details. Lux should be able to google it.

12. Well this is of course an hipothetical case though i recall reading astronauts using though i cant find it

I dont believe in conservation of angular momentum for this very case:

A counter rotating giroscope offers resistance to be spinned in certain axes

Then you use it to hold on and create momentum

13. Originally Posted by luxtpm
Well this is of course an hipothetical case though i recall reading astronauts using though i cant find it

I dont believe in conservation of angular momentum for this very case:

A counter rotating giroscope offers resistance to be spinned in certain axes

Then you use it to hold on and create momentum
You are mistaken in this case. If you have two counterrotating flywheel on the same axis, the system does not offer any resistance to being turned on any other axis. That "fight" you feel when you try and twist a gyroscope is because you are trying to change its non-zero angular momentum. Two counter-rotating gyroscopes have zero net angular momentum and you will not get the Same "fight" if you try and turn the system.

14. thanks that clarifies yet theres another case which confuses me:

which is the solution to this spinning skate dancer:

A)she decreased the radius of the weights so she increases rotational speed
B) she just changed the plane of rotation so rotational speed remains constant
C) both are posible it depends on how she follows the flow

if you get it wrong due to probability reasons you will get a negative score

15. Originally Posted by luxtpm
A counter rotating giroscope offers resistance to be spinned in certain axes
No, this is where your misunderstanding lies. A gyroscope does not offer resistance, it rotates freely while the spinning disk in its center retains its orientation. There is no resistive force, thus it cannot be used to create momentum.

16. Originally Posted by luxtpm
thanks that clarifies yet theres another case which confuses me:

which is the solution to this spinning skate dancer:

A)she decreased the radius of the weights so she increases rotational speed
B) she just changed the plane of rotation so rotational speed remains constant
C) both are posible it depends on how she follows the flow
I don't really know what your question actually is - I presume it is why the skater rotates faster when she draws in her arms. That is because the radius of the weights has decreased.

17. well my point is tht the skate dancer coud have reached position 2 in two different manners:

pulling the weights or changing the plane of rotation

in both cases final position is identical

so do you apply to this case conservation of momentum or not

18. Originally Posted by luxtpm
well my point is tht the skate dancer coud have reached position 2 in two different manners:

pulling the weights or changing the plane of rotation
I don't know what that means. The axis of rotation is the same in both positions; only the radius has changed.

19. check this video i made , i hope posting it doesnt meand this sent to garbage:

20. What was the point of the video?

21. to show that the position 2 of the skate dancer can be achieved by either pulling the arms or change the plane of rotation

22. Originally Posted by luxtpm
to show that the position 2 of the skate dancer can be achieved by either pulling the arms or change the plane of rotation
And....?

The axis of rotation doesn't change so all you are doing is changing the radius.

23. what if the skate dancer moves the arms as in my video?

do you still think she is changing radius?

24. If you put one arm up and one arm down, you bring both weights closer to the axis of rotation and speed up the rotation. If you hold your arms out sideways, you take the weights further away from the axis of rotation and slow it down.

25. Originally Posted by SpeedFreek
If you put one arm up and one arm down, you bring both weights closer to the axis of rotation and speed up the rotation.
exaclty and that doesnt happen in my video

26. Actually it does, but since the horizontal plane is wobbling, and the axis of rotation is contantly changing due to the varying center of gravity (It is after all suspended on a thread, a much different situation that that of a skater spinning over a fixed point), it seldom lasts a full rotation. It speeds up and slows down. When you measured the times of each rotation, what was the precise timing of each rotation? How much variation was there?

27. Originally Posted by MeteorWayne
Actually it does,
IT CANT im not giving energy imput, it keeps a constant linear speed as conservation of energy predicts

maybe whats going on in the video its wild precession?

28. Originally Posted by luxtpm
IT CANT im not giving energy imput, it keeps a constant linear speed as conservation of energy predicts
Your video just seems to show a bar wobbling about on the end of a string. How could you measure anything based on that? How did you measure the velocity? How did you change the orientation of the bar? How did you measure the angle and/or the radius of rotation? Why do you think this "experiment" (it hardly deserves the word) means anything?

29. well i believe in conservation of energy so i know the linear speed is constant

30. The two do not follow. Since the mass distance from the center of rotation is constantly changing in your "experiment" since energy is conserved, the rotatational (not liner) speed is constantly changing as well.

So what were the results of your measurements of the rotation times? (See post 25)

31. well i didnt measure

the point is what happens if the skate dancer pulls the weights in as in my video

everybody was assuming the skate dancer doubles her rotational speed if she halfs the radius of the weights, but my video proves that doesnt have to be the case necessarily

32. Originally Posted by luxtpm
my video proves that doesnt have to be the case necessarily
As has been pointed out, your video doesn't prove anything.

Why not? Well, as you said:

Originally Posted by luxtpm
well i didnt measure
Therefore you have no data to support your claim.

33. We KNOW the skaters rotation speeds up when they draw in their arms and legs. Your experiment is obviously flawed.

Watch, and learn.

34. Also, the centrifugal governor has used this for centuries.
Governor (device) - Wikipedia, the free encyclopedia

35. Originally Posted by luxtpm
well i didnt measure

the point is what happens if the skate dancer pulls the weights in as in my video

everybody was assuming the skate dancer doubles her rotational speed if she halfs the radius of the weights, but my video proves that doesnt have to be the case necessarily
Your video is not analogous to the skater pulling her arms in. For one, the rod hanging from the string is free to pivot on all axes. Where with the skater will assume that once she moves here arms to the new positions, she holds them there. We also assume that the friction between her skates and the Ice is enough to allow her to move the weights without losing her footing. The point being that the axis of rotation does not change.

In your video, there are a number of thing happening. For one you are suspending the two weight from that string in the middle of the rod. If you just let it hang there gravity will shift the arrangement so that the weights balance. if the weight are equal this will make the rod parallel to the ground. When you try to spin the weights as you do in the video, gravity tries to get the weights to balance, while at the same time the weights try to spin along the axis you spun them. The gravity working on the weights applies a torque to the system, which causes the axis of rotation to precess. The weights "wobble". Again, this is due to the freedom given in this situation for the axis of rotation to shift, a freedom not allowed with the skater if she is stay on her feet. (to be strictly correct, the axis does shift a little, the skater and and the Earth shift just the tiniest amount when she moves her arms, But since the weight of the Earth is so much more that that of the weights, this shift is all but unmeasurable.)

36. well i still dont see it:

imagine the skate dancer is told to relax the arms united by a beam jesus style while holding some weights

wouldnt then happen exactly as in my video and precession contradict conseraqvtion of angualr momentum?

37. No it would not. Your experiment is flawed for the reasons listed in the posts above, you made no measurements, so it wasn't an experiment at all, just a youtube type video.

38. Originally Posted by luxtpm
well i still dont see it:

imagine the skate dancer is told to relax the arms united by a beam jesus style while holding some weights

wouldnt then happen exactly as in my video and precession contradict conseraqvtion of angualr momentum?

NO. You keep neglecting that the weights in your video are not a closed system. They are effected by torque caused by the gravity of the Earth. You need to consider the whole closed system in order to account for all the angular momentum. Any apparent non-conservation of angular momentum is due to failing to take into account changes in the angular momentum of the Earth itself.

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