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Homework Help: Relative Velocity of Stone?

  1. Sep 17, 2014 #1
    1. The problem statement, all variables and given/known data
    A stone is dropped from a balloon at an altitude of 300 metres. How much time is required for the stone to reach Earth if:
    (1) Balloon is ascending with a velocity of 5 m/s,
    (2) Balloon is descending with a velocity of 5 m/s
    (3) Balloon is stationary
    Disregard air resistance.

    2. Relevant equations
    [itex] x=x_0+v_0t-1/2gt^2 [/itex]
    [itex] v=v_0-gt [/itex]

    3. The attempt at a solution
    I have no problem with question 3 but I fail to understand how the ascend or descend of balloon affect the stone. I assumed that it affected the initial velocity of the stone but I can't seem to find the formula for time.
  2. jcsd
  3. Sep 17, 2014 #2


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    Staff: Mentor

    What's the initial velocity of the stone when it's dropped from a moving platform? ("Dropped" means it's simply released).
  4. Sep 17, 2014 #3
    The question is about how to understand the initial velocity. When the balloon is moving upward (ascending) the object will have an upward initial velocity upon releasing, of course with the same magnitude as the balloon ascending. and vice versa in the case of downing.
  5. Sep 17, 2014 #4
    Is it the same as the velocity of the platform itself?
  6. Sep 17, 2014 #5
    Yes, as I said above.
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