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How to find the height where two same marbles have the same velocity?

  1. Aug 26, 2007 #1
    1. The problem statement, all variables and given/known data

    How do you find the height where two same marbles have the same velocity when one marble is dropped from the Earths atmosphere (g=9.8) and the other is dropped from the Moons atmosphere (g=1.6)? Both are dropped from the same height

    2. Relevant equations

    kinematic equations

    3. The attempt at a solution

    I have absolutely no idea where to start...
     
  2. jcsd
  3. Aug 26, 2007 #2

    learningphysics

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    Is that the question exactly?
     
  4. Aug 26, 2007 #3
    yes.
     
  5. Aug 26, 2007 #4

    learningphysics

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    Are they saying the velocity is the same as they hit the ground?

    That's a strange question... because if they are dropped from exactly the same height, then when they hit the ground they'll both always have different velocities because the g's are different... unless the height they are dropped from is 0...
     
  6. Aug 26, 2007 #5
    yea that was what i was wondering...no it doesnt say that.
    maybe the velocity is the same right when they are released?
    the only possible reason i can think of...
     
  7. Aug 26, 2007 #6

    learningphysics

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    Do they give any other numbers... like the height at which they're released?
     
  8. Aug 26, 2007 #7
    no they dont. they just say at height 'H'.
     
  9. Aug 26, 2007 #8

    learningphysics

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    Hmmm... that gives the impression that the answer can be given in terms of H... do they also say velocity V or anything like that?
     
  10. Aug 26, 2007 #9
    no, since they ask for the velocity in one part of the problem...
     
  11. Aug 26, 2007 #10

    learningphysics

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    Do they give drag or air resistance for this question?
     
  12. Aug 26, 2007 #11
    F=-kv

    and k= 0.1854
     
  13. Aug 26, 2007 #12

    learningphysics

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    Ah... ok, now I think I understand... a marble is dropped from a height on earth... due to air resistance it reaches its terminal velocity before hitting the ground... when it hits its terminal velocity, the net force on it is 0... so its acceleration is 0 at that point... and it stays at that terminal velocity...

    On the moon, there's no atmosphere... so there's no air resistance to think about.

    So, first find the terminal velocity of the marble on earth... then find the height that the marble needs to be dropped from on the moon, so that it reaches that velocity just as it hits the ground.
     
  14. Aug 26, 2007 #13
    ohh okay thank you. I think I got the terminal velocity...but I'm having trouble finding the height the marble needs to be dropped from the moon. What velocity should I use?
     
  15. Aug 26, 2007 #14

    learningphysics

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    The terminal velocity that you get for the earth marble.
     
  16. Aug 26, 2007 #15
    ahhh okay! thank you! would that just be my answer?
     
  17. Aug 26, 2007 #16

    learningphysics

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    Yeah, the terminal velocity would be your answer for that part.
     
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