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I need help to find the distance of the freefalling objects when one hits the ground.

  1. Jul 13, 2012 #1
    1. The problem statement, all variables and given/known data
    The two objects are dropped from 60 m height. One is dropped having an initial velocity of 20m/s and another with 15 m/s. What is the distance of the objects when the first one hits the ground if there is no air resistance.


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jul 13, 2012 #2

    Doc Al

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    Re: I need help to find the distance of the freefalling objects when one hits the gro

    What have you done so far? What kinematic equations describe the position of an object as a function of time?
     
  4. Jul 13, 2012 #3
    Re: I need help to find the distance of the freefalling objects when one hits the gro

    D= iV * T + a*t^2/2 I think thats the formula to be used.
     
  5. Jul 13, 2012 #4

    Doc Al

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    Re: I need help to find the distance of the freefalling objects when one hits the gro

    Good. So how can you apply this to solve for the time it takes for the 20 m/s object to reach the ground?
     
  6. Jul 13, 2012 #5
    Re: I need help to find the distance of the freefalling objects when one hits the gro

    Plugin the values for first object and calculate t.
     
  7. Jul 13, 2012 #6
    Re: I need help to find the distance of the freefalling objects when one hits the gro

    Would this work? 2(D-iV)/a = t
     
  8. Jul 13, 2012 #7
    Re: I need help to find the distance of the freefalling objects when one hits the gro

    No, how did you get this?
     
  9. Jul 13, 2012 #8

    Doc Al

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    Re: I need help to find the distance of the freefalling objects when one hits the gro

    No.

    Done right, you'll end up with a quadratic equation.
     
  10. Jul 13, 2012 #9
    Re: I need help to find the distance of the freefalling objects when one hits the gro

    Then how can I isolate t?
     
  11. Jul 13, 2012 #10
    Re: I need help to find the distance of the freefalling objects when one hits the gro

    Show us the equation you get after you plugin the values.

    For solving quadratic equations: http://en.wikipedia.org/wiki/Quadratic_equation
     
  12. Jul 13, 2012 #11
    Re: I need help to find the distance of the freefalling objects when one hits the gro

    60=20t + 4.9t^2
     
  13. Jul 13, 2012 #12
    Re: I need help to find the distance of the freefalling objects when one hits the gro

    Are you allowed to use g=10 m/s^2? That would make calculations a lot easier.
     
  14. Jul 13, 2012 #13

    HallsofIvy

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    Re: I need help to find the distance of the freefalling objects when one hits the gro

    You have two different objects, with two different initial velocities, each involving [itex]t^2[/itex]. Once you put the initial velocities in, you will have two different quadratic equations. What are those equations?
     
  15. Jul 13, 2012 #14

    Doc Al

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    Re: I need help to find the distance of the freefalling objects when one hits the gro

    Good. Now you can put it into standard form and solve it using the quadratic formula (or using whatever method you like).
     
  16. Jul 13, 2012 #15
    Re: I need help to find the distance of the freefalling objects when one hits the gro

    I got 2 as T for the first one.
     
  17. Jul 13, 2012 #16

    Doc Al

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    Re: I need help to find the distance of the freefalling objects when one hits the gro

    Good!

    Now see if you can figure out where the other object (the one thrown with the slower speed) is at that time. Use the same formula, but with different data.
     
  18. Jul 13, 2012 #17
    Re: I need help to find the distance of the freefalling objects when one hits the gro

    Ok I got the answer, the distance is 10.
    Thanks a lot.
     
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