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Homework Help: Momentum Problem with Time?

  1. Jan 28, 2010 #1
    1. The problem statement, all variables and given/known data
    A body of mass 50 kg is allowed to fall freely under the action of gravity. Calculate the momentum it possesses after 5 seconds


    2. Relevant equations
    P=mv
    F=ma


    3. The attempt at a solution
    I'm thinking we're really looking for how force affects momentum here? Since (F=ma) and acceleration is just how velocity changes over time would we use:
    F = m * v/t
    and then replace v with a =
    F=(50)(9.81)/5 = 98.10 J

    Is this correct?
     
  2. jcsd
  3. Jan 28, 2010 #2

    radou

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    As you mentioned, momentum is defined as p = mv. v varies with t. What is the velocity of the mass during its' free fall?
     
  4. Jan 28, 2010 #3
    v = vi- gt =
    0-(-9.81)(5) = 49.05 m/s
    this is assuming initial velocity was 0 which i dont think we can really assume.
    Would we then insert that value:
    F = m * v/t
    F=(50)(49.05)/5 = 490.5 J
     
  5. Jan 28, 2010 #4

    radou

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    Of course we can. :)

    So, you got the velocity after 5 seconds right.

    Now, you only need to calculate the momentum, not the force. p = mv. you have the mass, and you have the velocity after 5 seconds.
     
  6. Jan 28, 2010 #5
    So then we would do:
    P=mv = (50)(49.05) = 2,452.5 J
    correct?
     
  7. Jan 28, 2010 #6

    radou

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    Yes, you got it right.
     
  8. Jan 28, 2010 #7
    thank you!
     
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