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Homework Help: Word Problem with Kinematics Equation

  1. Feb 23, 2014 #1
    An object thrown up from a cliff 10 m/s reaches a velocity of 20 m/s [down] as it lands. If acceleration due to gravity is 9.8 m/s2, what is the object's displacement? How long did it take for the object to land from the time it was thrown?

    2. Relevant equations
    I'm not sure if all of the below are relevant, or perhaps some equations are missing.

    Δx = v0t + 1/2at2
    (displacement = initial velocity x time + 1/2 acceleration x time squared)

    vf2 = v02 + 2aΔx
    (final velocity squared = initial velocity squared + 2 acceleration x displacement

    3. The attempt at a solution
    t1 = v/a = 1.02 s
    t2 = v/a = 2.04 s
    t1 + t2 = 3.1 s

    Not sure how to find the displacement, but the answer key says it's -15m. How do I find it?
  2. jcsd
  3. Feb 23, 2014 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You can use either of your two equations to solve for the displacement. If you use the first, do not round off the time to 3.1 seconds.

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