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Cofused with this Momentum problem

  1. Dec 15, 2005 #1
    19. Incorrect In a combustion engine, gas is burned. The resulting explosion produces a force that drives the pistons in the engine. The force of the explosion on the piston is due to the change in momentum of the gas molecules. A 0.4-g sample of gas produces a force of 2400 N in an explosion that lasts 10e-3 seconds. What must be the speed of the gas molecules?

    My answer: 6 x 10e3 m/s
    The correct answer: 3 x 10e3 m/s

    I used the impluse force = change in momentum forumla

    ft = mv

    2400(10e-3) = .0004(x)
    and solved to get 6e3 m/s

    What am I missing?
  2. jcsd
  3. Dec 15, 2005 #2


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    the molecules don't stick to the piston, they bounce off.
    so the change of momentum is ( p_f - p_i ) double the final momentum.
  4. Dec 15, 2005 #3

    I get that...but how is the answer 3e3 m/s ?

    could you put some numbers into the explanation, because conceptually i'm not getting why the change the velocity is 3e3??
  5. Dec 15, 2005 #4


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    suppose the molecule is above the piston, calling upward as positive.
    v_i is negative "v" , but v_f is positive "v".
    The change of momentum is m times 2"v".

    The change of momentum of the molecules is the negative of
    the change of momentum of the piston (I expect you DID get this).
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