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Homework Help: Recoil velocity

  1. Dec 18, 2007 #1
    1. The problem statement, all variables and given/known data

    First: My course has sig figs disabled. We are recording all that is shown in calculations. Its my instructor's choice, not mine. Please do not lecture on sig figs. I know already. ;)
    A person stands in a stationary canoe and throws a 4.85-kg stone with a velocity of +7.73 m/s at an angle of 35.3 ° above the horizontal. The person and canoe have a combined mass of 102 kg. Ignore air resistance and effects of the water and find the horizontal recoil velocity (magnitude and direction) of the person and canoe.

    3. The attempt at a solution

    horizontal velocity = velocity of 7.73m/s * cos 35.3 = 6.3087
    mass of stone 4.58kg * horizontal velocity of 6.3087 = 28.894 kg m/s
    28.894 kg m/s = 102kg * recoil velocity
    28.894 / 102 = recoil velocity = -0.28327 m/s This is incorrect. What have I done wrong? The negative is for the opposite direction movement.
  2. jcsd
  3. Dec 21, 2007 #2

    Shooting Star

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    Homework Helper

    Do you know the answer? Also, check the exact wording of the problem once more, in case something has been missed.
  4. Dec 23, 2007 #3


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    Presumably, either 4.85 kg. or 4.58 kg. is correct for the mass of the stone. (I also want to stay away from someone who can throw an 11 pound stone that fast...). Somewhere you have a copying error, which may explain why the computer didn't like your answer...
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