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Homework Help: Hard Motion Question- Help?

  1. Feb 1, 2008 #1
    Hard Motion Question- Help???

    I am really stuck on this motion question, can anyone help please????:confused:

    "A balloon is 30.00m above the ground and is rising vertically with a uniform speed when a coin is dropped from it. If the coin reaches the ground in 4.00 seconds, what is the speed of the balloon?
  2. jcsd
  3. Feb 1, 2008 #2

    Ben Niehoff

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    Gold Member

    So, the coin has an initial height of 30 m, and an initial velocity of v. It has a constant acceleration of -9.8 m/s^2. Find the time it takes to fall to a height of 0 m, in terms of your variables v and t. Then plug in 4.00 s for t, and solve for v.
  4. Feb 1, 2008 #3
    which motion equation do i use?
  5. Feb 1, 2008 #4
    solve for the initial velocity of the coin,,since it is equal to the constant velocity of the balloon, try

    0 = 30 + Vi * 4 - .5g(4^2)

    thing to know is that if the balloon is traveling , say 500 m/s when the coin is dropped....the coin is actually dropped AT 500m's...so its initial velocity when you insert it into an equation is 500m/s,, NOT 0 as you might assume.
    though thats irrelevent here, thought i might say. so that you understand the purpose of the question really
  6. Feb 1, 2008 #5
    are you saying the coin when dropped is would be travelling the same speed of the balloon in an upwards direction?
  7. Feb 1, 2008 #6
    yes, ya and its acceleration is changed from 0 to -9.8 when you put it in the equation because of gravity...because of course it will be hitting the ground soon
  8. Feb 1, 2008 #7
    I got 12.1 from that equation, but what is that number?
  9. Feb 1, 2008 #8
    the coins initial velocity once it is released from the balloon. = 12.1

    which just so happens to be the balloons constant velocity, since when you initially dropped the coin from the balloon its initial velocity is = to the balloon velocity.
    so, Balloon-velocity = 12.1

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