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Homework Help: Velocity from a force time graph

  1. Apr 17, 2009 #1
    1. The problem statement, all variables and given/known data

    The figure is a graph of the force exerted by the floor on a woman making a vertical jump.
    At what speed does she leave the ground? Hint: The force of the floor is not the only force acting on the woman.

    2. Relevant equations



    3. The attempt at a solution

    I am completely lost with this question. First I derived her mass from the 600N from the graph. Which was about 61.22kg

    Then I calculated the Impusle from the area underthe graph, but where from? All the area or minus the forces of gravity? then I used:

    Impulse= m(delta v)

    But Im not getting anywhere :(
     

    Attached Files:

  2. jcsd
  3. Apr 17, 2009 #2

    Astronuc

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    Starting with this: Impulse= m(delta v)

    Then delta v = Impulse/m, and delta v = v - vo, and vo = ?
     
  4. Apr 17, 2009 #3
    Im guessing Vo is zero becasue she is jumping straight up?

    Do I calculate Impulse to be the entire area under the graph or just a certain segment under the graph?
     
  5. Apr 17, 2009 #4

    Astronuc

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    Correct. Standing she starts are rest.

    Weight which is a force, does not provide for acceleration. One has to consider the force in excess of the weight.
     
  6. Apr 17, 2009 #5
    Thanks for your replies.
    So since F=mg, and I derived mass from 600/9.8. does that mean that the impusle is the area of the graph from anything that is >600n?
     
  7. Apr 17, 2009 #6

    Astronuc

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    The 600 N is her weight, when she is not moving, so the force in excess of 600 N goes toward accelerating her mass.
     
  8. Apr 17, 2009 #7

    Astronuc

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    Another way of looking at this problem is to convert F(t) to a(t), since a(t) is just F(t)/m, however, one has to look at Fnet(t), which is the difference between Ftotal(t) and mg.
     
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