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Kinematics Problems

  1. Oct 16, 2007 #1

    H^w

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    1. The problem statement, all variables and given/known data
    A person jumps from a window into a fire safety net. The person drops 15.0 meters, stretching the net 1.0 meter. What is the average deceleration before coming to rest?


    2. Relevant equations
    All of the constant acceleration equations should apply here.


    3. The attempt at a solution
    Well it would be sad to call this an attempt. But here goes.

    I collected everything I knew about the motion.

    The person is in free fall, therefore I know that Vminimum is at t=0

    V 1=0 m/s a=9.8 m/s and the total displacement I believe should be 16.0 meters because the net stretched 1.0 meter whether or not thats relevant I'm not sure.

    So where to go from here is my real problem.

    Thanks
     
  2. jcsd
  3. Oct 16, 2007 #2

    Kurdt

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    Assume the person starts from rest. You will have to find the speed the person is travelling just before they hit the net. Then you will need to find the time taken to travel one meter when they hit the net. Then you can work out the average acceleration.

    [tex]a=\frac{\Delta v}{\Delta t} [/tex]
     
  4. Oct 16, 2007 #3

    H^w

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    What Im not understanding is I only have a, and initial velocity (V=0).

    So how do I solve for t and V final?
     
  5. Oct 16, 2007 #4

    Kurdt

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    Assume the jumper has an initial velocity of 0m/s. One can use the following kinematic equation to find their velocity 15 meters below the jumping point.

    [tex] v^2 = u^2 + 2as [/tex]

    Now you know that at the 15 meter point the person hits the net and comes to a velocity of 0 m/s in 1 meter. You can employ another kinematic equation involving the initial and final velocity and the distance to find the time taken. Then you will have enough information to work out the average acceleration.
     
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