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Thread: Momentum

  1. #1 Momentum 
    Forum Professor leohopkins's Avatar
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    Lets say I am standing on a very long plank of wood in outer space. The plank of wood is moving at 20 km/h towards alpha centurai. I am standing on the plank so I too am moving at 20 km/h. Now suppose I start walking along the plank at 10 km/h AWAY from the direction that the plank is travelling. How fast am I now moving ?


    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  3. #2  
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    this is relative.
    relative to the plank, you're moving 10km/h.
    relative to the universe, you're moving at 20km/h minus 10km/h.

    we're already on a "moving plank" called the earth, which is zooming around in the universe at... well i don't remember the speed, but its tremendous.
    and earth is partof an even larger relativity, namely the suns relativity,
    but even the suns relativity isn't the end, the sun itself is part of star clusters, which themselves are part of galaxies.
    so even when "standing still", you're moving at a tremendous speed through the universe.

    relativity is all about being correct within a frame of reference.


    when you have eliminated the impossible, whatever remains, however improbable, must be the truth
    A.C Doyle
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  4. #3  
    Forum Professor river_rat's Avatar
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    Relative to the universe? What exactly do you mean by that?
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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  5. #4  
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    Hey leo, what's the planck length...
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  6. #5  
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    Very long
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  7. #6  
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    dejawolf is right in one regard: The question "how fast am I moving" is meaningless without reference. I think the question suggests Alpha Centauri as the reference point, i.e. it should be "how fast am I moving w.r.t. Alpha Centauri".

    Maybe the question would be more interesting if the plank were to move towards A.C. with the speed of light, and you were walking towards A.C. along that plank with 10 km/h. How fast would you then move w.r.t. A.C. ?
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  8. #7  
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    Well that wouldn't be realistic, as the plank CAN'T move at the speed of light and so the paradox created by that is pointless.
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  9. #8  
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    Yeah and as everyone knows, Walking the plank usually results in an untimely death, so don't try this at home folks.
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  10. #9  
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    Although in this case there would be no sharks to eat you.
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  11. #10  
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    And no pirates to force you off it.
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  12. #11  
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    Okay, so, does the system (plank and yourself) have more energy when you are moving on the moving plank or when you are standing still on the moving plank ?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  13. #12  
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    If you start moving on plank, and your motion is due to friction with the plank then the energy will be the same, in propelling yourself forward you gain energy but the plank will lose it, overall no change, see newtons laws of motion...
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  14. #13  
    Forum Professor leohopkins's Avatar
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    How will the plank loose energy ?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  15. #14  
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    You didn't look up ol Isaac did you...

    Suppose it's not you on the plank but a ruddy great cannon, when you fire a shot the cannon and the plank will recoil, that's how the plank loses energy BUT if the shot slams into a wall at the end of the plank then the energy will be restored - net gain = 0.
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  16. #15 Leohopkins is correct 
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    Mr leohopkins explained very clearely abt relativity. He is correct..
    I am reddaiah.
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  17. #16  
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    Well that wouldn't be realistic, as the plank CAN'T move at the speed of light and so the paradox created by that is pointless.
    Not realistic? Someone walking on an endless plank in outer space should not be realistic? I do that every day.

    Alright, so let's say the plank moves at speed of light minus 5 km/h towards Alpha Centauri. As it moves along, you suddenly enter the universe (back from a coffee break in another universe) in the center of the plank, going at the same speed as the plank. The plank is made of iron, and your cool space shoes have magnetic soles, just strong enough to give you some friction. Now you start walking toward Alpha Centauri. As Megabrain points out, the plank will slow down, but ever so slightly, because it has almost infinitely more mass (and hence momentum) than you. You, on the other hand, because you're so light (or are you?), speed up to 10km/h, relative to the plank, in direction of Alpha Centauri. Unrealistic, yes. Physically possible? What's the problem?
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  18. #17  
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    Well with your numbers you are basically still moving towards AC at C-5km/h as the correction is too small to notice!

    Its easier so assume that the plank is moving at 90% of the speed of light relative to an observer on earth and you use your fancy rocket shoes to speed you up to 50% of the speed of light relative to the plank. Then the observer on earth would measure your speed relative to himself to be approx 97% of the speed of light.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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