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Gyroscope - I just don't get it.

  1. Nov 1, 2004 #1

    quasar987

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    I don't understand why a rotating gyroscope whose rotation axis is inclined with respect to a vertical axis does not fall to the ground dues to gravity as fast as the non-rotating giroscope.

    Thanks for your replies.
     
  2. jcsd
  3. Nov 1, 2004 #2
    Until a more elaborate answer comes along, consider that a red dot on the outer edge of the spinning mass wants to go as straight as possible, just like a red dot painted on a moving mass in free space. The red dot (the local mass it represents) has inertia, rotationnal inertia in this case, which tends to keep it going in the initial manner.
     
  4. Nov 1, 2004 #3

    quasar987

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    Ok so could it be that the answer is: An ideal rotating giroscope will never fall to the ground due to gravity because it has a vertical component of angular momentum and since the gravitationnal force exerts no vertical (y) torque , the vertical component of angular momentum must be conserved. So the thing will rotate about the vertical axis due to the components of gravitationnal torque in the x-z plane but will not fall to the ground because that would mean losing vertical angular momentum.

    On the other hand, the non-rotating gyroscope having no angular momentum at all will fall to the ground WHILE gaining a little bit of angular momentum in the x-z plane because of the gravitationnal torque being exerted for the time of the fall.
     
  5. Jan 11, 2005 #4
    I came to this forum for exactly the same question. quasars last note is confusing at best.

    A spinning gyroscope overcomes gravity, How?

    "rotating gyroscope whose rotation axis is inclined with respect to a vertical axis"

    consider only two directions UP and DOWN.

    The VERTICAL spinning top attempts to "throw" itself in all directions on the plane os spin (horizontal). Because these forces are balanced it is stable and does not fall.

    But when inclined on an angle we introduce another force "gravity", which increases the downward force generated by the top, and reduces the upward force generated by the top

    The top is now out of balance and **should** come crashing down to earth.

    But it does not ? Why not.

    (laymans terms please, I am smart enough but have never done higher levels of math)

    Chris.
     
  6. Jan 11, 2005 #5

    quasar987

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    Too bad no one answered to my last note, I still don't know if that's the reason or not. If it is, then I still find the phenomenon peculiar. I'ts as if one law of physics (conservation of angular momentum) was "more important" or had priority over the other (gravity). :grumpy:

    What are those forces generated by the top you talk about?
     
  7. Jan 11, 2005 #6
    Hi quasar.

    The forces I speak of are in effect the red dot spoken of earlier.

    Imagine a weight attached to the spinning top, when the top spins the weight attempts to pull the top in the weights current direction. in other words its out of balance pulling the top in the direction of the weight.

    Without the weight the tops own mass is doing the same thing but in every direction at once, ie balanced. (only at 90 degrees to the spinning axis).

    but on a top that is not perpendicular to gravity. the force created by the spinning top now MUST be out of balance. this you understand otherwise you would not have asked the original question.

    Its as if the rotating force of the top 'ignores' the force of gravity

    In fact if you attempt to change the angle at which the top is leaning the top will resist the change.

    To feel that resistance pick up a barrel vacuum cleaner while it is running. it kicks and bucks in your hand if you attempt to move it into another plane.

    Chris H.

    Note:
    The upward and downward only relate to the fact these forces are not parallel to the surface of the earth (gravity) in an inclined top.
     
  8. Jan 12, 2005 #7

    DaveC426913

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    A spinning top does *not* defy gravity.

    You must support one end of the top, else it will fall to the ground just like every other object. If you put the supported end on a weigh scale, it will weigh exactly as much as if it weren't spinning at all.

    So, the only mystery is why its *orientation* appears to defy gravity. (Spin the top and put its axis of revolution horizontal. Place one end of the spinning top on your finger. It looks like it should fall off your finger, but does not. That's the mystery.)

    Consider a non-rotating top on your finger. If you try to support it with a finger under only one end, it will fall (obviously). But before it falls, it has to *rotate* slightly until its support "clears" your finger. Only then can it proceed to the ground.

    The spinning top is merely resisting that tendency to rotate slightly. Since it won't rotate, it can't get its support off your finger. (But not that it still presses down on your finger with the same weight a non-spinning top does.)
     
  9. Jan 12, 2005 #8
    Ok
    I assume you mean "the long axis of the top must rotate around your finger before falling"
    given that assumption, for a top which is not rotating about its own axis gravity applies normally. and the top topples as one would expect.
    (a more precise description would be something like it must rotate around its point of contact on the surface of your finger)

    Then you go on to state
    This is the crux of the matter, you pass it off with no explanation.

    WHY does the top resist to rotate. even when it is already rotated off the vertical
    A top spinning with its axis off the vertical will hold the angle of rotation until the top loses enough speed, then it topples. Why?

    I have attached an image to further explain what my dilemma is
     
  10. Jan 12, 2005 #9
    Yea, i just "seems" to defy gravity, but it doesn't. Using a top as an example: Take a top that's not spining and mark a spot on the outer edge of it. If you tilt the top slightly towards spot, there will be more downward force on that part of the top so it will start to tilt and fall that direction. Rotate the top 90 degrees. Where the spot is on the top no longer has that downward force, but it still has the veocity from it so the top will lean towards the new position of the spot. Rotate it another 90 degrees and the same thing happens. Keep doing it over and over again and eventually the top will be leaning towards that new direction. Keep going and it will be leaning in the opposite direction of the initial tilt you gave it. So, the top will just wobbles instead of falling over.

    That's the best explination I can come up with.
     
  11. Jan 12, 2005 #10
    I'll try again with the attachments
    broken into two due to size
     

    Attached Files:

  12. Jan 12, 2005 #11
    Hi Romper take a look at the images, (shame they didnt show) you will see what I mean

    your explanation works perfectly for a top with a vertical axis. its the off vertical that is of greatest concern
     
  13. Jan 12, 2005 #12
    P.S. By inclining the top we are moving its centre of gravity until it is no longer directly above the point of contact on the surface. the centre of gravity is now suspended, without support. yet it behaves as if it is unmoved.

    I would like to experiment but don't have the nec. equipment
    exp. 1. spin top in vertical position. weigh it
    exp. 2. spin top in inclined position. weigh it

    Are they the same? if so why?, if not why?

    Where is the centre of grav for both of the above? the centre of mass or the point of contact with the surface.

    Has the centre of gravity moved down the top to the point of contact. thus no topple?
     
  14. Jan 12, 2005 #13
    Actually, I think my explinations is flawed. Gyroscopes don't rotate that direction, they rotate in the opposite direction of spin. The way they work is shown in your second drawing.

    If the axis is tilted and the gyroscope is spinning, then the upward force on one side will hold that side up, while the extra downard force on the other will make the gyroscope fall towards that side. This is exactly why tops wobble, because its axis does start to move out of the vertical position. If it didn't, then precession would never occur.
     
  15. Jan 12, 2005 #14
    I just had an interesting thought. Is gravity 'sticky'

    What I mean by this is gravity attaches itself to the downward spin of the gyroscope. and the gyroscope is thrust toward the floor.

    Now the spin of the gyroscope causes that gravity particle (stay with me here) which has attached itself to the down side, is now being thrown upwards.

    This upward moving gravity particle is now opposite the new downward particle which attached itself to the now downward side of the top these two gravity particles cancel each other out thus the overall effect of gravity is nil.

    :-)

    Of course I see many flaws in this argument. but the idea of sticky gravity is interesting. at least in that it has momentum and its direction can be changed.

    Hey its no weirder than 11 dimensions!!!!
     
  16. Jan 13, 2005 #15

    cepheid

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    This is very hard to explain without diagrams, but I don't think the fanciful explanations we're getting ourselves into are really necessary. Consider an analogue in translational motion first:

    A ball on a string, at rest, will accelerate toward you if you grab the end of the string and pull on it, making it taut.

    Now, what about a ball with some initial translational momentum? You pull on the string in a direction perpendicular to the ball's initial velocity, and voila! It suddenly begins to move in a circular loop about your hand. But it's distance from your hand does not change! You are pulling it toward you...why is it not responding as in the first case? Is it defying the tension force? Obviously not...we know that the tension force acts as a centripetal force. A simple analysis of the vectors reveals why the object with some initial translational momentum "behaves" differently under some applied force than an object at rest.

    The same is true in the rotational case. Consider a rotating bicycle wheel. with a string attache to the centre. When it has no angular momentum (it is not spinning), and you try to hold in in the vertical orientation, it will fall back down:


    |
    | <--- string
    |
    | / <--- wheel
    /

    side view of the wheel, hanging from a string, almost vertical. Why does it flop back down? In the position shown above, the centre of mass is no longer directly below the pivot point (where the string is attached), so the gravitational force exerts a torque that rotates the wheel back down to this position:

    _______|
    _______| <----string
    _______|
    _______|
    _____------ <---- wheel

    Now, what happens when the wheel is spinning (ie it has some inital angular momentum pointing roughly to the right, for example. The very same torque (which points into the page), has a very different effect...that angular momentum vector does not decrease (because the torque is perp. to it), and it changes direction...so the wheel remains vertical even after you let go! But it precesses (rotates about an axis // to the string in this case):

    |
    | <--- string
    |
    | / <--- wheel (now spinning, so it remains in this orientation)
    /

    So, in a direct analogy to the previous example, an object with some initial angular momentum reacts differently (and in a non-intuitive way) to an applied torque than an object initally non-rotating does. For those of you (probably everyone) who couldn't make heads or tails of my stupid diagrams, please check out the video here, and read the article...it explains gyroscopes quite well:

    http://science.howstuffworks.com/gyroscope1.htm
     
    Last edited: Jan 13, 2005
  17. Jan 13, 2005 #16

    DaveC426913

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    I have never seen math like this. Gravity does not attach itself to other forces. Gravity acts on the object as a whole.
     
  18. Jan 13, 2005 #17

    DaveC426913

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    (Centre of gravity? You mean centre of mass.)

    The centre of mass has support. It is supported by the rigid metal structure of the top. One end of the top is resting on my finger.


    Look, I'm not suggesting it is inherently stable in that position, I'm merely trying to get you to realize that gravity is not the force to worry about here. There is no doubt why the object doesn't fall due to gravity, the big question is, why does the object resist rotation.




    They will weigh the same. I'll bet 10 million dollars on it. Or to put it another way, if you manage to use a gyroscope to reduce the mass of an object, I'll raise and give you all the money you need, since you will have revolutionized technology. (Imagine jumbo jets, powered by gyros, weighing nothing at all).
     
  19. Jan 13, 2005 #18
    Hmm, I just re-read my second post and it doens't make much sense. My first explination looks right. I guess that's what happens when I get too tired, heh. Thanks for the explanation Cepheid.
     
  20. Jan 13, 2005 #19

    BobG

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    DaveC426913 is giving you the correct description of what is happening. It's not that gravity isn't still pulling down the same as it always was. The gyroscope isn't resisting gravity anymore than a block sitting on a table is resisting gravity's urge to pull it to the floor.

    Gravity is a weak force (just keep telling yourself that all the way down the side of the cliff). You can resist it with a pretty small magnet. But the force of gravity has to have some effect on the gyroscope.

    It still creates a torque that tries to rotate the gyro off the string, but it can't overcome the angular momentum of the gyro. It will cause the gyro to precess around the string.

    This has some good info on gyros and how they work. I started at the top of the gyro section, but the specific info you want is buried down a few different branches.
    http://hyperphysics.phy-astr.gsu.edu/hbase/gyr.html
     
  21. Jan 13, 2005 #20

    Excellent so now I understand what precession means. I couldn't pick it up from my reading of the dictionary.

    I always thought the reason the top rotated around its pivot was simply due to the friction of the pivot on its support. I did not realise that this was precession. thats why it was not shown in my drawings its the missing 'force' that counter balances the WEAK force of gravity.

    DaveC426913wrote...
    aren't the two synonymous? it was always called the centre of gravity to me.

    Now I know this is going to sound STUPID! it does to me now, but when I spoke of sticky gravity. (oh god I feel stupid admitting this). I was talking about inertia. but in my infantile way I called it sticky gravity if I had thought with a reasonable number of synapses instead of the few I put on it somewhat jokingly, I would have realised that for every section of the top it develops Inertia. that Inertia is then displaced as the wheel rotates and its is now acting in a slightly different direction.

    This happens continously as the top rotates, and you have precession.

    Thats what I meant by the momentum of gravity can change direction. (stupid description, I apologise to those with sensitive minds)

    momentum of gravity = inertia.

    Ok I fully understand why a gyroscope does not fall.

    Thanks to all for this wonderful exercise of discovery.

    P.S. DaveC I'd love to take the money off you. but can it wait for my next extraordinary Gaff
     
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