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Matthew's Problem Zone #2

Discussion in 'Nerd Out Zone' started by PsychoticLeprechaun, Feb 18, 2014.

  1. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    Hello there all!

    This is a series of questions designed to tax the brain!

    The rule of the series is simple:
    Don't use the internet for help - it ruins the fun!

    So, for our second question, we have one that is slightly physics-based, but not anything that logic and intuition cannot break through (even without a conventional basis in physics education)

    [hr]

    ««« Take a candle and place it inside a glass tube, such that wind does not affect the flame. Now place it on a carousel and have the carousel rotate (either direction). In which direction will the flame tend? »»»

    [hr]

    This one should hopefully require a little more though than the last one, and as such I will refrain from providing the answer for a longer period. So post your thoughts below!
  2. jessenic

    jessenic Web Developer

    • Dev Member
    If the flame tends at all, I'd say it tries to tend along the diameter line of the carousel away from the center point.
  3. vesa

    vesa Kerbalnaut [4/50]

    • Member
    the candle is constantly being accelerated towards the center of the carousel, so i would say that the flame goes away from the center of the carousel.
  4. Arctic

    Arctic Giant Robot Advocate

    • Tester
    Depends whether it's accelerating.
  5. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    Arctic, I'd ask you, what is the definition of acceleration?

    Vesa and Jessenic, I'd just ask you to explain why it might be the case that it tends outwards?
  6. jessenic

    jessenic Web Developer

    • Dev Member
    Because of a force that I can't say in English, in Finnish it's something like "Middle escape force"
  7. Restless

    Restless Terraformer

    • Member
    Is the tube that's been placed over the candle closed at the bottom, or can air still circulate?

    If it's open won't some of the air still hit the rim of the tube at the open bottom (on the sidefacing away from the turning direction of the carousel) and shoot up inside the glass? I can picture the flame swirling about as a result but couldn't even begin to guess in which direction, maybe clockwise if the carousel is going anti-clockwise?

    If it's closed the flame would die from lack of oxygen but that's too easy so I assume it's not closed.

    If somehow the glass could completely block out any air from the outside yet still prevent the flame from suffocating... hm... I was never any good at physics, I even failed high school physics. I'm guessing since the flame is in a closed space the air pressure and current shouldn't change much even if the carousel is moving. Imagining the carousel is going anti-clockwise I would imagine the air inside the tube would seem to circulate clockwise, but it's actually the tube that's turning with the carousel, not the air inside. I don't think it would cause the air inside to circulate faster would it? Not if the carousel is going at a steady speed.

    Depending on the speed of the carousel there might be some kind of observable centrifuge-like effect on the flame making it want to lean out directly away from the centre of the carousel.

    I think it's becoming clear that I have no idea what I'm rambling about so I'll just stop there.
  8. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    Jessenic:

    You perhaps mean the tendency to move outwards from the centre of the circle? Your explanation is close to right, but your answer derived from it isn't quite right.

    Restless:

    The running out of air is of no concern to us; we are looking at the tending of the flame - it's motion. Therefore the latter of your three suppositions is effectively correct, that the flame does not go out and that the wind (and therefore any eddy current) does not affect it. Your suggestion it would lean out seems to be explained similarly to how Jessenic did also, so my response would be the same, near correct explanation (but incomplete) but flawed conclusion from that.
  9. Asimov

    Asimov Festina lente. [#32]

    • Member
    It will tend straight up - the same as if it's not moving. The tube will act like a venturi and the movement will create a low pressure zone inside the tube causing any air that enters it to move down the walls and then back up the center.
  10. Causeless

    Causeless narcissistic, machiavellian and sadistic

    • Moderator
    The wind would probably make it go out.
  11. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    Asimov, that is indeed true in part, flames do tend up because of that, however it will tend in a given direction other than vertically.

    Cause, there is no wind! It wouldn't go out ;P
  12. Asimov

    Asimov Festina lente. [#32]

    • Member
    Ok, so when you say glass tube with no effect from wind, you're basically saying it's a sealed tube as far as the air movement is concerned? You're just looking for centripetal/centrifugal effects?

    If so, it's probable that the flame moves toward the axis of rotation.
  13. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    Why would it necessarily move towards the axis about which it rotates?
  14. Asimov

    Asimov Festina lente. [#32]

    • Member
    A rotating frame of reference is just like being in a gravity field. Just like when you swing a bucket of water on a rope.
  15. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    Ah, but when you rotate a bucket, in which direction does the water tend? If it tended to the axis of rotation, surely you would be soaked by the contents of the bucket, instead those contents remain in the bucket.
  16. Asimov

    Asimov Festina lente. [#32]

    • Member
    So you're saying that in a gravity field the water in the bucket would act differently?

    The flame rises due to air density differences in a gravity field. A rotating frame of reference (seen from the inside) is indistinguishable from a gravity field. (With one exception that doesn't apply here, or applies in a very minimal way: Coriolis effects.)
  17. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    Rotating systems aren't the same as a gravitational field, as a gravitational field is effectively constant (at least on the surface of the Earth, even up to the ISS and a fair way beyond). Rotation however results in an acceleration such that the motion of the body is always at a tangent to the circle it follows. In such a case, acceleration must always be perpendicular to the motion of the body towards the axis of rotation. So yes, there is a force inwards (if that is why you said that the flame (or body) would tend to lean towards the axis of rotation) - the centripetal force - but yet, as you said, there is an apparent (not real) centrifugal force by the water (tending out from the axis of rotation) when one swings a bucket such that it remains in its position at all points on the circle (even when gravity would have you believe it would fall) which would therefore have it that the flame would lean away.

    So why is it that, as you said initially, the flame leans away? (I am not confirming or denying that it does - just to be clear)
  18. Asimov

    Asimov Festina lente. [#32]

    • Member
    Air density is higher toward the outside and lower toward the axis.

    And while there are differences between a rotating frame of reference and a gravity field, if you're the one sitting in a boat on the water in a bucket, without being able to see outside the bucket, there are no tests you can do to prove which you're in. (Other than the Coriolis effects which, depending on the scale of the circle, may or may not be noticeable.)
  19. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    Yeah, that is true, taken from that reference you could describe it in that way! The Equivalence Principle is a wonderful one, and a damned painful one too (General Relativity is a pain)! ;P

    Okay, so just to be sure, why is it that the air density means that the flame would move towards the axis of rotation, when all the previous parts suggested that it would move away?

    Edit: I must bid you farewell for tonight! I will catch up with any and all posts tomorrow!
  20. Asimov

    Asimov Festina lente. [#32]

    • Member
    To put it simply: Heat rises. In this case "rise" is toward the axis.

    If you say that the flame goes toward the outside, it's just like saying that if you light a candle on your desk the flame would point down.
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