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Basic physics questions

  1. Feb 17, 2007 #1
    Hey all,


    I was wondering if someone could explain how to tell whether V = - or +?

    I'm getting answers right using the formula

    V^2 = sqrt(Vo^2 + 2ay)

    but I'm getting positive answers. How do you tell whether the velocity is in the + or - direction?


    Sample Question:

    A hot-air balloon is rising upward with a constant speed of 2.50m/s. When the balloon is 3.00m above the ground, the balloonist accidentally drops a compass over the side of the balloon. How much time elapses before the compass hits the ground?


    I got V = 8.1m/s but the right answer requires using -8.1m/s.

    thanks for any help :)
     
  2. jcsd
  3. Feb 17, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    They expect you to know that a falling object hits the ground with negative (downward) velocity!

    Both answers are consistent with the given initial conditions of acceleration and velocity. But only one of those answers is the one you're looking for. The other answer is the velocity the object would have to leave the ground with to end up 3 m high with the given speed. (In other words: What goes up, comes down. :wink: )
     
  4. Feb 17, 2007 #3
    ah ok. that makes since... thanks!
     
  5. Sep 17, 2007 #4
    hi,

    i saw the thread and wanted to know how we would actually go about answering the questions it asks. I figured out how to solve the problem but I cant figure out how many seconds it would take for the compass to drop. please help.

    thanks,

    Z
     
  6. Sep 17, 2007 #5

    Doc Al

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    Staff: Mentor

    What have you figured out so far?
     
  7. Sep 17, 2007 #6
    i basically have the same example, but the values given to me are different. the problem i have is:

    A hot-air balloon is rising upward with a constant speed of 2.20 m/s. When the balloon is 3.50 m above the ground, the balloonist accidentally drops a compass over the side of the balloon. How much time elapses before the compass hits the ground?

    i was able to solve as far as the other student, but then I get stuck. any suggestions?
     
  8. Sep 17, 2007 #7
    The best equation for you to use is

    [tex]s = s_0 + v_0 t + 1/2 a t^2[/tex]

    Solving for t is then a quadratic. (Why does this topic have so many views?)
     
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