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Homework Help: [Theory] What's the initial velocity of this falling object?

  1. Sep 27, 2015 #1
    • Member warned to use the formatting template
    So if you have an object in a hot air balloon that it travelling upwards at constant velocity, and this object falls out of the hot air balloon, is its initial velocity 0 or is it the same as that of the balloon?

    Thanks in advance!
  2. jcsd
  3. Sep 27, 2015 #2


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    What do you expect?
  4. Sep 28, 2015 #3
    I think this question is based on refrence point.As with respect to hot air ballon the body is at rest with intitial velocity then being equal to zero,while if you consider earth as refrence point then the object has some initial velocity which is equal to that of ballon given that refrence is earth.
  5. Sep 28, 2015 #4


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    When the problem states that a hot air balloon is "travelling upwards at constant velocity", that already tells you that the default reference frame in use is not that of the hot air balloon. The natural assumption is that the default frame is one in which the ground is at rest.

    We need to wait for the original poster to tell us what he thinks about the initial velocity of the dropped object relative to the ground.
  6. Sep 28, 2015 #5


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    Initial velocity relative to what? A key point to this question is that "velocity" can only be given relative to some zero point. The velocity of the object relative to the balloon and the velocity of the object relative to the ground are two different values.
    Last edited by a moderator: Sep 28, 2015
  7. Sep 28, 2015 #6
    I strongly agree that this question is only answerable through the observer's point of view.

    Let's say that the situation you speak of only takes into account the moment the object falls. In this case, the object's initial velocity from the point at which it is dropped is zero. In summary, this is because the observer knew nothing before the instance of the object's descent from the air balloon. And if we do take into account the object's velocity from the moment the hot air balloon begins to ascend, then the initial velocity of the object is the same value as the air balloon's velocity. In summary, this is because the observer knew that that object was travelling at a velocity parallel to the air balloon from the very moment he or she observed it.

    Ultimately, the initial velocity of the object seems to be dependent upon the parameters of the observer's frame of reference.
  8. Sep 28, 2015 #7


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    Do not invent difficulties that you do not have to.

    If the problem tells you that the balloon has an upward velocity then the balloon has an upward velocity. If you can find a frame where this is not true then that is not the frame of reference implied by the problem. It does not matter what inertial frame of reference you assume that the problem is using. It does not even matter whether that frame is inertial. The balloon is moving upward in that frame because the problem specifies that it is doing so.

    No. That is incorrect.
  9. Sep 29, 2015 #8
    Yah you are right sir
  10. Sep 29, 2015 #9
    My apologies. And looking back at my post, I now see the difficulties you speak of.
    Okay, I wrongly assumed that we were talking about specific frames of reference.
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