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Please verify this notion regarding . momentum

  1. Apr 12, 2013 #1
    Please verify this notion regarding..... momentum

    Hello. Could someone verify this notion about momentum.... thanks

    here is info:
    - The conserved closed system here, is a vehicle and the planet (via the road attached)
    - A stationary vehicle has no momentum
    - A moving vehicle has some momentum

    here is point to verify:
    A moving vehicle has created a quantity of momentum (forward direction for vehicle) AND it has created a quantity of momentum (opposite direction for planet)
    * So... the total quantity of momentum is exactly half each?
    As p=mv
    * So... the vehicle will have very small m and large v quantity
    * So... the planet will have large m and very small v quantity

    I wish to know if I can correctly state...... the total quantity of momentum is half each (for vehicle and planet)

    [this is linear momentum and no other factors effect the closed system]

    much appreciate thoughts, cheers stephenn
  2. jcsd
  3. Apr 12, 2013 #2


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    Staff: Mentor

    You've pretty much on the right track here, but it is not correct to say that the total amount of momentum is "half each". Momentum is a signed quantity (that is, direction matters) so you should say that the total momentum is still zero:

    Vehicle's momentum we'll call P; we get this by multiplying a small mass times a large velocity. Now the earth's momentum is -P; we get this by multiplying an enormous mass times a very small negative velocity. Total momentum is the momentum of the earth plus the momentum of the vehicle, P+(-P) = 0.

    Doing it this way, the total momentum is conserved: zero before and zero after.
  4. Apr 12, 2013 #3
    Thanks for reply.... and workings.... I understand that the phrase of half and half doesn't carry with it, the direction factor (one being opposite to the other).

    I still wish to test "worded" / "spoken" notions, that will hold true:
    so for the same notion:

    In the closed system, mentioned..... should this hold true?:
    Any momentum present (for vehicle and planet), will be identical in quantity, but apply in opposite directions.
  5. Apr 12, 2013 #4

    Doc Al

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    Staff: Mentor

    Better to say: The change in momentum of each will be equal and opposite.
  6. Apr 12, 2013 #5
    haha, I was prodding and poking at my sentence to get it more concise.... got the equal and opposite bit.... your "change" and neat phrasing gets a thumbs up, thankyou. :smile:
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