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Homework Help: Help with projetile motion using Energy Theorem

  1. Oct 23, 2005 #1
    I just wanted some one to check over my answer because i am doing my hw online and it keeps telling that my answer is close, and that i have made a rounding error or used the wrong sig figs.

    The problem is
    A baseball is thrown from the roof of a building of height 22.1 m with an initial velocity of magnitude 12.1 m/s and directed at an angle of 53.1o above the horizontal.

    What is the speed of the ball just before it hits the ground.

    Ok i split the problem into two parts. From the point he threw the ball to when it reaches its max height and then from its max height to the point where it hits the ground. For the first part its going to be

    Ki + Ui = Kf + Uf
    the intial potential energy is going to be zero.
    I used this equation to find the max height it reached and i got 4.78m.

    Then i used the equation again except this time it has to final potential energy. I used 26.9 as my height and solved for the final velocity. And i got 22.9m/s. But the thing keeps telling me i am close. I tried to find the answer using kinematics and i got the same answer. Can some one tell me if i am doing something wrong.

  2. jcsd
  3. Oct 23, 2005 #2
    Think about what you just wrote. If he is 22.1 meters up on a building, and throw the ball up at an angle, how can the maximum height reached be equal to 4.78meters? 26.87 sounds about right. As for the speed before it hits the ground, it is influenced by projectile motion, which means it has a component of velocity in the +x direction as well. Take that into account by using sqrt(x^2+y^2) to find the magnitude of the net resultant velocity just before impact. Hopefully that will find your mistake.
    Last edited: Oct 23, 2005
  4. Oct 23, 2005 #3
    But i am assuming that the point where he threw the ball is the origin, and the the distance below is -22.1m.
  5. Oct 23, 2005 #4
    But most likely the question assumes the orgin as the ground, not the top of the building, unless it explicitly says so.
  6. Oct 23, 2005 #5
    Well, actually, reading it a second time, it seems that all you care about is the final velocity before impact. If you did your y component right, try factoring in the x-component, I bet thats what you neglected. And see if it pans out or not.
  7. Oct 23, 2005 #6
    Yes you are right, i for got to take the x-component into consideration. I was just giving the y-component of velocity.

  8. Oct 23, 2005 #7
    cool, no problem. Are you using webassigns.net? my friend uses that for his online physics homework, just curious.
  9. Oct 23, 2005 #8
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