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I think this is a physics question, might seem dumb but it's been bugging me all week

  1. May 14, 2003 #1
    Hi all, I'm totally new to this board, I signed up specifically to ask one question and I think one of you people might be able to explain it to me.

    On the bus the other day my friend and I noticed a fly flying around the bus, the bus was stopped. when the bus started driving again the fly maintaned it's position. How come the fly didn't end up at the back of the bus even though it was hovering in mid air and wasn't resting on a surface???

    Anyone have any clue what I'm talking about??

  2. jcsd
  3. May 14, 2003 #2


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    Because the air in the bus was also (mostly) moving along with the bus itself.

    - Warren
  4. May 15, 2003 #3
    I didn't think it'd be that simple, well there ya go then, thank you.


    oh yeah, while I'm here, (a bit more serious this time), how can light tarvel in both waves and particles ???
  5. May 15, 2003 #4
    about your fly question
    you should consider the the bus is close system
    just like the earth
    now the earth is moving at a high speed....but why u didn't fall down
    about the light
    actually light is electromagnetic wave
    it travel in a wave and with a light speed C
    just sometime it will show the particle property
  6. May 15, 2003 #5
    On the old forum someone asked a similar question about a helium balloon. They said that when the bus started moving, the balloon instead of heading for the back of the bus as expected, moved forward toward the front of the bus. I think the concensus was that the air pressure in the back raised when the bus moved forward damming the air toward the back (I don't remember if the bus windows were open or not).

    So the fly may have actually been fighting the force of the air pressure to move it toward the front of the bus.

    Although it is a mostly closed system (barring ventilation and open windows), so its motion should remain relative to its position inside of the bus.
  7. May 15, 2003 #6
    As for the fly question, i've always wondered the same thing, and i just assumed that, as a previous reply states, that it was because the bus was in essence a closed system. i would certianly like to know if that was not the case.

    As for the light question, i disagree that light is simply a wave...i think that it is both a wave and a particle, although that is a fairly radical theory, and just a theory. i can't explain that theory, however, so do ask me to...i happen to be great at understanding physics as long as i'm not the one teaching it!
  8. May 15, 2003 #7
    About light, the unsatisfying answer is that you can't observe the wavelike and particle like nature of light at the same time, so you can't really expalin how it can be one and the other at the same time. You can do an expirement that shows the light diffracts like a wave, or you can do an experiment that shows that the photons can bang into other particles like a particle. But you can't do an expirment that shows both at the same time. This is called "wave particle duality".
  9. May 16, 2003 #8
    yes ....the light is wave and particle
    but it travel with electromagnetic wave
    this can prove with the Maxwell's eqaution
  10. May 16, 2003 #9

    Ivan Seeking

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    I know this one. Someone noticed this with cigarette smoke in a car. When you accelerate smoke in the car goes forward. The key is that the smoke and the balloon are both buoyant. The cooler heavier air goes to the rear forcing the warmer air forward. [A slight modification to the answer I was getting here].
  11. May 16, 2003 #10
    I never noticed that with smoke in a car before, I'll have to check that out later.

    ok regarding the light question, light particles are photons aren't they? do they have any weight, they must for gravity to be able to effect it???
  12. May 16, 2003 #11

    Ivan Seeking

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    Nothing with mass can travel at speed C. However, we can infer a mass for the photon by considering the momentum and speed of the "particle". This implied mass agrees with other observed properties of light. But, it can't have mass. The problem here is that mass is energy. The photon has no rest mass [I have always found this statement amusing since I have never seen a photon at rest!] but a moving photon does have energy...and E = MC^2. The main thing is that a difference does exist between these two ideas - the mass of a photon and the energy of a photon - but the difference can get confusing. We normally do not consider light to have mass. To really understand why light follows the path it does in a gravity field requires an understanding of General Relativity.

    Is light a wave or particle. I think the best answer is that it depends on how we look. When we look for particles, we force the particle-like nature to appear. When we look for a wave, we force the wave-like nature of light to appear. Light [and many other things it turns out] is something that can act like either waves or particles, but is not distinctly either. Beyond that, we quickly get into trouble if we try to define what we mean too much. Out puny little brains just can't handle the idea very well. That’s why we use mathematics instead of language to “talk” in physics. We can treat things like light as a mathematical object, and we can then work very precisely, often without really understanding what the equations are telling us.
  13. May 17, 2003 #12
    It is pretty simple to think about it, but if you go deep into the problem it is only a bit more complicated. What the fly experienced (and you did as well) was the effects of general relativity (more or less because gravity really wasn't too important here. From your inertial reference frame, you were moving nowhere, but from a person on the street's reference frame, outside of the bus, you were moving 45 mi/h (let's say for example). If you threw a ball 20 mi/h in the bus, at your reference frame, it is moving 20mi/h, but to the person on the corner of the street, it moves at 65 mi/h. I hope I didn't confuse you... or myself!
  14. May 17, 2003 #13
    As for the fly question,I think its just the viscous force which accelerated it forward when bus strated again.When the bus is not moving, the air inside is also stationary and thus fly experience no viscous force.When the bus starts moving,the air inside the bus can be seen as a stream (or wind) from the inertial refernce frame(i.e. road).Thus fly is getting dragged alongwith like a piece of paper in a stream of water(or for that matter air).

    For the second question,the thing is that you can't talk of particles or waves per se when you got to plank levels.The light is not a pure wave like a water wave because single photons experiments have been done successfully.Similarly nor you can say that its like everyday matter particles we see because light follows superposition principle,the paradigm of wave-nature.Thus,light has got both properties---wave and particle.The important question thus arises when it shows as a wave and when as a particle and the answer seems that it depends upon how you do the experiments(or WHAT are you INTERESTED in measuring).

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