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Wierd observations

  1. Oct 27, 2004 #1
    I was riding the subway yesterday on the way back from a concert, and i saw a girl holding a helium balloon and when the train started moving the balloon moved foward with the train, i thought that because of the acceleration and the inertia the ballon would move back, but it actually went foward. When the train stopped also that balloon moved back not foward like everything else in my section of the train. What was going on there???
    Also what is Fire...i mean what state of matter is it?
  2. jcsd
  3. Oct 27, 2004 #2
    LOL I don't find that train thing as wierd as you seem to. When the train started the air around it got disturbed in such a way so as to move the baloon forward. If you think in terms of aerodynamics it makesperfect sense.

    Fire is a visual release of energy during an exothermic process. When bonds are broken between atoms (during the burning of a fuel or other organic substance) energy is released as light and heat.
  4. Oct 27, 2004 #3


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    I don't think you need to invoke aerodynamics - there's no lift involved here! It's simply a matter of buoyancy.
  5. Oct 28, 2004 #4


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    Fire is radiation plasma, radiation being heat and light, and plasma, being the state of matter. :biggrin: Yes, I myself find trains's physics a most intersting thing.
  6. Oct 28, 2004 #5
    Fire is the first of the 4 elements! It's represented by a somethingsomething-hedron, but I'd forgotten what that somethingsomething is.
  7. Oct 28, 2004 #6


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    Why does a helium balloon float in the first place?

    The heavier air (atomic mass of nearly 60) displaces the lighter Helium (atomic mass of 4).

    Same thing happens when the subway accelerates. All the gases, like everything else, are pushed to the back of the subway, but the heavier air displaces the Helium. The helium balloon is floating the same as it ever was - away from the accelerating force, whether it be gravity or the force from an accelerating train.
  8. Oct 30, 2004 #7
    There is no measureable difference between gravitational field and acceleration. If your train accelerates, think about its interior as a place with artificial gravity. Air is heavier, ballon is lighter, that's all.

    Helium balloon in an accelerating vehicle (moving faster, turning) is a beautiful example if you are talking about relativity and the nature of gravity.
  9. Oct 30, 2004 #8
    Fire is matter in a gaseous form heated enough to release light energy. So, in general, fire is carbon or a carbon compond in gaseous form that is hot enough to release light energy(in particuar, infrared light).
  10. Mar 29, 2010 #9
    I know this is an old post, and dont ask me how I found it, but i feel that no one as answered the question clearly.

    I don't know what fire is, but the balloon in the train went forward because the density of the air changes when the train suddently moves forward. When the train accelerates, the density of the air is not the same at the back of the train than it is at the front of the train. So when the train moves forward, the air is, as suspected, condenced to the back of the train. So you have more air in the back of the train than at the front of it... Helium, being lighter than air, wants to get the hell out of the there, it cannot stand the pressure, so it moves forward. So it appears, counter-intuatively, to violate the laws of motion when in fact it does not. Density, in this case, is stronger than motion.

    Sorry for the bad english, It's not my first language.

    see ya!!!
    Last edited: Mar 29, 2010
  11. Mar 29, 2010 #10
    Sir, you are wrong. I had a course of aerodynamics, and I know that when Mach number is smaller than ~0.3 you can neglect compressibility of air, Ma=0.3 being in air standard conditions 360km/h I doubt air inside of train was traveling at that speed, there is no compression of air, air moves in opposite direction of train's acceleration due to its inertia, pushing lighter baloon forward just like in the situation when you go backwards you pull air behind you forwards, but in baloon's case its lighter than air. The surplus of air that would be behind is not compressed but its flowing out through cavities in doors etc. or flowing back forward, and lack of air in front of train is comming inside the same way, or is being sucked back from the end of train, train is not hermetic.

    I am also sorry for any spelling mistakes, and for thread necromancy
    Hello forums:) I obviously did not register to post this, stay tuned for an avalanche of my questions soon
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