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Homework Help: Motion and speed

  1. Aug 5, 2010 #1
    1. The problem statement, all variables and given/known data
    A stone is droped from a balloon going up with a uniform velocity of 5m/s.if the baloon was at 50m high when the stone was dropped, find its height when the stone hits the ground?
    find the time for

    X(distance)=-50m, u(initial velocity)=5m/s a(acceleration)=-10m/s*s

    2. Relevant equations

    please show the the calculaion and methods to solve this problem
    rather than this method
    also if there is any other simpler way to solve this question?

    3. The attempt at a solution
    i am not able to solve by using X=ut+1/2at*t
    i tried the method shown in the book but i m not able to solve the above equation
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Aug 5, 2010 #2


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    Homework Helper

    That's the correct method
    Make sure you got the signs correct for each quantity, pick a direction (ie up is +ve)

    As a starting point, imagine if the rock was simply dropped - work out the time for that, then you know your answer should be a bit larger than this,
  4. Aug 6, 2010 #3
    Take care of two things:

    1. You have to find the height of the balloon.
    2. Is the height of the balloon constant? NO! It changes with time. So, you have to find out the height of the balloon at some specific time! What is that time?

    Now, I ask another question. What question needs to be solved first? 1 or 2?

    For 2, all you have to do is use the s = ut + 0.5at^2

    u = -5m/s, a = 10m/s^2, s = height of the balloon when the stone was thrown (+50 or -50?)

    Find time.

    Then for 1, there's no acceleration! So which equation will you use?
  5. Aug 6, 2010 #4
    Another way to solve the question would be to setup equations for both the cases, with s1 and t1 as the unkowns.

    Then solve them simultaneously through the methods learned in ninth standard!

    Solve the question both ways in your notebook - simultaneously and sequentially!
  6. Aug 6, 2010 #5
    Think about mgb_physics' suggestions.

    He's giving you a physical picture. I gave you a way to get the answer - that's a technique. What he said is, our case will take longer time than the case where the rock would have been simply dropped.

    Why's that so? Think about it, draw a simple diagram in which you contrast the two situations. Then tell me what you think!

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