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Homework Help: Force (Newtons Laws of Motion)

  1. Jul 9, 2009 #1
    1. The problem statement, all variables and given/known data
    For a birthday gift, you and some friends take a hot-air balloon ride. One friend is late, so the balloon floats a couple of feet off the ground as you wait. Before this person arrives, the combined weight of the basket and people is 1220 kg , and the balloon is neutrally buoyant. When the late arrival climbs up into the basket, the balloon begins to accelerate downward at 0.59m/s2 .What was the mass of the last person to climb aboard?

    2. Relevant equations

    3. The attempt at a solution
    do I need to find the net force before and after the last person's arrival?then compare those 2 equations to get the last person's mass? could somebody give me a clue how to approach this kind of a problem?

    Thanks in advance.
  2. jcsd
  3. Jul 9, 2009 #2


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    On the air balloon there are two forces, weight (downwards) and upthrust (upwards) .

    So yes, you need to compare before (which will give you the upthrust) and then after to get the person's mass.
  4. Jul 9, 2009 #3
    so would the 1st equation be??
    upthrust (F_u) - Wt of the basket & people (W) = mass (m). acceleration (a)
    F_u - 1220 * 9.81 = 1220 * 0.59
    Therefore F_u = 12919.8 N

    would the 2nd equation be??
    F_u - (1220+mass of last person)g = (1220+mass of last person)a

    am I in right way ........??? if not could you please help me?

    Thanks again...
  5. Jul 9, 2009 #4


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    I think [itex]F_U -W =0[/itex] since before the other person jumps into the basket it is just hovering. The second equation is correct.
  6. Jul 10, 2009 #5
    The first equation is Fu-Wt=0 ,because at the beginning the acceleration is zero, because "the balloon is neutrally buoyant" at the beginning.
    Last edited: Jul 10, 2009
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