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Homework Help: Question on Free Fall motion

  1. Nov 22, 2013 #1
    1. The problem statement, all variables and given/known data

    The original question is:
    A ball is shot horizontally off the edge of a cliff 35 m high. The ball lands 20 m from the base of the cliff.
    a. How long was the ball in the air?
    b. What was the initial velocity of the ball?

    2. Relevant equations

    I believe these would be helpful.
    x - Δx = vt - (1/2)at^2
    x - Δx = Vot + (1/2)at^2

    3. The attempt at a solution

    To find time, I have this:
    20 = 0t - (1/2)(-9.8)t^2
    20 = 4.9t^2
    t = 2.02 sec

    which I then plugged into the second equation to find Vo

    20 = Vo (2.02) + (1/2)(-9.8)(2.02)^2
    20 = 2.02 Vo - 20
    40 = 2.02 Vo
    Vo = 19.802 m/s

    This unfortunately appears to be in error, but I am unsure where I went astray. I think it's something about the incorrect equations being used, or my not fully understanding the problem.
  2. jcsd
  3. Nov 22, 2013 #2
    The ball is being shot horizontally with certain velocity, not given.
    The ball is "dropped" vertically from rest and travelled distance given.
    You can find time travelled by the ball to the base.
  4. Nov 22, 2013 #3
    Hi - just a quick question to clarify. When you say I can find the time by the ball to the base, are you saying that the x is irrelevant (so I should try setting my equation equal to 0 instead of 20?).
  5. Nov 22, 2013 #4
    x is the product of time and velocity. No forces acting horizontally, so acceleration is irrelevant.
    y is given with initial velocity and acceleration(since there is force acting on it, gravity). From this you can find the time.
  6. Nov 22, 2013 #5
    Alright. I'll give that a try. Thank you.
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