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Linear and projectile motion?

  1. Nov 14, 2011 #1
    1. The problem statement, all variables and given/known data
    A ball is thrown downward from a tower 65.0 m tall. If the ball takes 1.30 seconds to hit the ground, what was its initial velocity?

    The answer my teacher provides is 43.6 m/s, but I am having a problem reaching that answer.

    2. Relevant equations
    Possibly Vi = Vf - aT? This is the equation I am trying to use, but the final velocity is not given, I don't think.

    3. The attempt at a solution
    First, I tried Vi = 0 + 9.8 x 1.3, but obviously, it doesn't work. So whether there is information missing or I am using the wrong formula, I don't know. Any help would be appreciated!
  2. jcsd
  3. Nov 14, 2011 #2


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    Homework Helper

    The values you have are:

    x - displacement - 65
    a - acceleration due to gravity - 9.8
    t - time - 1.3

    All motion and acceleration is down, so I have made all values positive

    You want vi

    so you need the motion formula with out the final velocity. That is

    x = vi.t + 0.5.a.t2

    sub values into that and see how you go.
  4. Nov 14, 2011 #3
    Thank you! That seem to have worked. (: I knew there was something wrong with the formula I was using.
  5. Nov 14, 2011 #4


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    Homework Helper

    You should always identify those values you know, and the value(s) you want, to help in deciding which formula to use.
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