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Problem in equations of kinematics in two dimensions

  1. Sep 8, 2008 #1
    Problem in equations of kinematics in two dimensions..plz help

    Hi, I'm a first year physics student and am studying equations of kinematics in two dimensions and projectile motion. I was given the following question as homework to prep for my first exam this Wednesday. I'm pretty solid on the subject but for some reason I am stuck on this problem:

    1. The problem statement, all variables and given/known data

    "A car drives straight off the edge of a cliff that is 54m high. The point of impact is 130m from the base of the cliff. How fast was the car traveling when it went off the cliff?"

    2. Relevant equations

    Vx = Vox + AxT
    X = 1/2(Vox + Vx)T
    X = VoxT + 1/2AxT^2
    Vx^2 = Vox^2 + 2AxX

    Vy = Voy + AyT
    Y = 1/2(V0y + Vy)T
    Y = VoyT + 1/2AyT^2
    Vy^2 = Voy^2 + 2AyY


    3. The attempt at a solution

    Based on the information given, I know my Delta X and Delta Y. However, no matter which equation I use to sovle for Vox (the speed of the car when it left the cliff) I always have 2 unknown values in my equation. If I knew Vx or T I could solve this. However, I know that it is solvable, my professor posted T and VoX. I am just totally stuck on where to take the first step in the problem.

    Any help is greatly appreciated.
    Bat.
     
  2. jcsd
  3. Sep 8, 2008 #2

    LowlyPion

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    Re: Problem in equations of kinematics in two dimensions..plz help

    Welcome to PF.

    You know the height, so you can calculate the time to impact.

    That times the horizontal velocity is known to be 130 m.
     
  4. Sep 8, 2008 #3
    Re: Problem in equations of kinematics in two dimensions..plz help

    Hrmm.. see this is where I'm confused. I only have a distance. I don't know my initial or ending velocity or the time it took to travel the 130m?
     
  5. Sep 8, 2008 #4
    Re: Problem in equations of kinematics in two dimensions..plz help

    I did try this route once:

    130m = 54mT, T = 2.41

    However, the professor gave the answer to the problem and has T = 3.32 sec

    I cannot figure out how she got this?
     
  6. Sep 8, 2008 #5

    LowlyPion

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    Re: Problem in equations of kinematics in two dimensions..plz help

    But you have more information that is not stated.

    You have gravity. It's easy to forget because it's always there.

    How long does it take the car to drop vertically in the gravity field off the cliff?

    That would be given by H = 54 m = 1/2 (9.8 m/s2) (t2)

    You can find these equations in your text book or from this link:
    https://www.physicsforums.com/showpost.php?p=905663&postcount=2
     
  7. Sep 8, 2008 #6

    LowlyPion

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    Re: Problem in equations of kinematics in two dimensions..plz help

    This equation just can't be right.

    You have distance = distance * time.

    The ratio of the distances in this case is simply not the time.
     
  8. Sep 8, 2008 #7
    Re: Problem in equations of kinematics in two dimensions..plz help

    I see now!

    I get 3.3197 sec for T.

    Damn how did I miss this... whats screwing me up is that I kept thinking I needed a Vo or V to get my Time.
     
  9. Sep 8, 2008 #8

    LowlyPion

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    Re: Problem in equations of kinematics in two dimensions..plz help

    Here is a link with common kinematic formulas.
    https://www.physicsforums.com/showpost.php?p=905663&postcount=2

    You look good to go now.

    Good Luck.
     
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