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A stone thrown straight up how much time before it reaches a certain height?

  1. Jan 2, 2007 #1
    Here is my question:

    A stone is thrown straight up with a speed of 15.0 m/s. How fast will it
    be moving when its altitude is 8.0 m above the point from which it was
    thrown? How much time elapses while the stone is reaching that height?
    (Is there one answer or are there two answers? Why?)

    ---

    I know that the stone goes up and then down again, so there could be two answers, but how exactly would I solve this problem?
     
  2. jcsd
  3. Jan 2, 2007 #2

    cristo

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    Staff Emeritus
    Science Advisor

    Hi and welcome to PF. In order for us to help you, you will need to show some working or your thoughts on the matter. Better still, as I presume this is a homework question, use the homework template
    I'll give you a hint. You should use the "kinematic equations." Try googling, and giving it a go; post your attempts and we will be able to help further.
     
  4. Jan 2, 2007 #3
    Oh, okay, thank you.

    Question:
    A stone is thrown straight up with a speed of 15.0 m/s. How fast will it
    be moving when its altitude is 8.0 m above the point from which it was
    thrown? How much time elapses while the stone is reaching that height?
    (Is there one answer or are there two answers? Why?)

    Variables:
    Vi = 15.0 m/s
    d= 8.0m
    a= -9.8 m/s/s
    Vf = ?
    t = ?

    Equation I used:
    d=vi(t) + 1/2 (a)(t)^2
    Vf^2 = Vi^2 + 2ad

    This is what I did;

    8 = (15)t - 4.9 (t)^2
    4.9t^2 - 15t + 8 = 0
    Solving that, I got 2.37s, and 0.688s

    For finding the final velocity (when at 8.0m);

    (15)^2 + 2(-9.8)(8)
    =8.26m/s

    Is this the right procedure? Because I'm not sure if I'm doing this correctly..
     
  5. Jan 2, 2007 #4

    Doc Al

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    Staff: Mentor

    Looks good to me. There are two answers since the stone passes the 8 m altitude both on the way up and on the way down.
     
  6. Jan 2, 2007 #5

    cristo

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    Staff Emeritus
    Science Advisor

    Correct; the two answers appear since the stone moves past the point s=8m on the way up and on the way down.


    Yes, that's correct.

    Edit: Sorry, late!
     
    Last edited: Jan 2, 2007
  7. Jan 2, 2007 #6
    Okay, Thank You! =)
     
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