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Conservation of momentum/KE

  1. Oct 23, 2013 #1
    In a Physics Q, (June 2010 OCR B Q 12) we have in part a.
    A hammer of mass 2.0Kg is used to set (a mass on a spring) into oscillation. The following observations are made.
    The hammer approaches with an upward velocity of 5.0 m/s
    There is a sharp click as the hammer hits the mass.
    the hammer leaves the mass with a downward velocity of 3.3 m/s

    (i) Why does the second observation suggest about total KEn is not conserved in the collision. (1)

    The mark scheme gives one mark for "Sound energy produced (at expense of KE)"

    Now I know that this is a standard explanation for the lack of conservation of the quantity 0.5mv^2. However, (I am asked by a student) sound carries momentum too does it not, so why does the observation not say that Momentum is not conserved.?

    My answer to this objection (sound carries momentum too) is that the sound carries in all directions and therefore the total loss in momentum is zero. However, I am interested in whether my answer is correct or is there a more sophisticated response?
    Last edited: Oct 23, 2013
  2. jcsd
  3. Oct 23, 2013 #2


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    That's pretty much it, actually.

    The total energy and momentum is always conserved, but if energy is transferred to the air, there is less energy in the motion of the hammer and mass.

    The same is true for momentum, but the total momentum transferred to the air adds up to zero; the momentum of the parcels of air pushed leftwards added to the momentum of the parcels of air pushed rightwards gives you a zero net added momentum.
  4. Oct 23, 2013 #3
    Yes that what I thought.
    When I was at school, the non-conseravtion of KE was demonstrated mathematically and I was never terribly happy with this as a "demonstration" since I figured that a maths demo merely demonstrated a limitation of the model.

    It occurred to me that it might be possible to set up some kind of experiment that demonstrated this by causing the sound emitted to be all in one direction. Thus momentum would not be conserved either.

    Just a thought.
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