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Problem on Kinematics

  1. Oct 13, 2012 #1
    1. The problem statement, all variables and given/known data

    A balloon is at a height of 81 m is ascending upward with a velocity of 12 ms1. A body of 2 kg weight is dropped from it. If g = 10 ms2, the body will reach the surface of earth in what time.


    2. Relevant equations



    3. The attempt at a solution

    I don't think mass is of any importance in this question. Hence, I tried to solve it this way:

    Since initial velocity = -u
    (s=ut+1/2at^2)

    81 = -12t+1/2*10*t^2
    Solving, we have t = 42seconds.

    However this is not the answer. Please explain how I solve this question.
    Thanks in advance
     
  2. jcsd
  3. Oct 13, 2012 #2

    cepheid

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    Hello physics kiddy,

    I think your signs are messing you up. You can orient your coordinate system however you want, it's up to you. But you have to pick a direction and stick to it consistently. In this case, let's choose upward to be the positive direction, and downward to be the negative direction. Then, the object's initial velocity (when dropped) is *upward*, because it is ascending along with the balloon at +12 m/s.
     
  4. Oct 14, 2012 #3
    I have assumed upward to be negative and downward positive to make things easier, however, I am not getting the answer...
     
  5. Oct 14, 2012 #4

    cepheid

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    Can you show the steps of your solution in more detail?
     
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