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Calculation of Kinetic Energy

  1. Dec 7, 2014 #1
    1. The problem statement, all variables and given/known data
    A body, which has mass of m = 1 kg , is dropped with a horizontal speed of vo = 20 m/s . What is its kinetic energy after 4 s ?

    2. Relevant equations
    Ek = mv2/2
    s = vot + gt2/2

    3. The attempt at a solution
    2qn5h6a.png
    Is it correct?

    P. S. There is a mistake: Ek = 3200 J
     
    Last edited: Dec 7, 2014
  2. jcsd
  3. Dec 7, 2014 #2

    NTW

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    It's wrong. First of all, you should add the velocities as vectors. And, in this case, those vectors are at 90º...
     
  4. Dec 7, 2014 #3
    after 4 seconds ,,
    It would have a vertical component of velocity equal to (v0-gt)"which is what you have calculated in your derivative , which is incomplete too , " and a horizontal component equal to (v0 cos [theta=0])(which you haven't included)

    Just find the magnitude of them and substitute the value of velocity in the kinetic energy formula
     
  5. Dec 7, 2014 #4
    OK, from your answers I understood that:

    vy = voy - gt
    So, after 4 s :
    vy = 0 - 10 m/s2 * 4s = -40 m/s

    vx = 20 m/s (since it does not change)

    v2 = (vy)2+(vx)2 = 2000 m2/s2

    v = 45 m/s

    Ek = mv2/2 = 1000 J

    Corrected
     
    Last edited: Dec 7, 2014
  6. Dec 7, 2014 #5

    NTW

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    You have forgotten g...
     
  7. Dec 7, 2014 #6
    What about now?
     
  8. Dec 7, 2014 #7

    NTW

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    Now it's right...
     
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