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Time and Velocity question

  1. Mar 19, 2014 #1

    a.k

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    1. The problem statement, all variables and given/known data
    A man using a hang glider who is descending at a speed of 8 m/s drops his cell phone from an altitude of 30 m.
    a. How long does it take the cell phone to reach the ground?
    b. What is the velocity of the cell phone just before it hits the ground?


    2. Relevant equations
    Vf^2=V0^2+2ay
    t=v-v0/a


    3. The attempt at a solution
    (-8m/s)^2+2(-9.8 m/s)(-30)

    I want to make sure I am using negative velocity and accel due to descending speed...
     
  2. jcsd
  3. Mar 19, 2014 #2

    a.k

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    64 m/s^2+(-19.6m/s)(-30m)
    64+588
    vf2=652
    sqrt(652)
    25.53
    Due to descending, it is -25.53 m/s

    t=vf-vi/-9.8 m/s^2

    =-17.3/-9.8=1.765
    t=1.77 seconds
     
  4. Mar 19, 2014 #3

    PhanthomJay

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    those answers look good. You can avoid confusion with the minus sign by assuming down direction as positive, but what you did was fine, although you did have to have the insight that the sq root of 652 was the negative root
     
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