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Homework Help: Rock thrown from cliff, find initial and final velocities

  1. Oct 16, 2014 #1
    1. The problem statement, all variables and given/known data
    I'm taking grade 12 U physics and I'm having a really difficult time understanding the answers given compared to the answers I arrive at.

    For example:

    The question reads:

    A rock was thrown horizontally from a 25.0 m high cliff and landed 15.0 m from the base of the cliff. Determine the initial speed with which the rock was thrown, as well as its final velocity.

    2. The attempt at a solution

    V1y = 0.0 m/s
    ay=9.8 m/s^2
    Displacement = 25 m

    Solve for y-component

    25m = (0.0 m/s)(change in time) + 1/2(9.8 m/s^2)(change in time)^2


    (change in time)^2 = 2(25)/9.8m/s^2
    change in time = Sqrt 2(25) / 9.8m/s^2
    change in time = 0.7215375318230077

    This is the answer I arrived at.

    My workbook however, says the answer is 2.26. Can someone please explain to me how my workbook arrived that that answer? I cannot move on in the question until I solve this.

    So far every example in my course provides the correct calculations but all of the final answers are completely incorrect.
  2. jcsd
  3. Oct 16, 2014 #2


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    Gold Member

    You did order of operations wrong. Work it out again from the first line and keep your parentheses.
  4. Oct 16, 2014 #3


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    Homework Helper

    Welcome to PF!
    You miss parentheses. (change in time)^2 = 2*25/9.8 = 5.102. Take the square root to get the time : The correct formula is (change in time) = Sqrt ((2*25) / 9.8)
    You took the square root of 50 and divided it by 9.8, which is wrong.

  5. Oct 16, 2014 #4
    Thank you both so much!! you're amaz-o

    My entire workbook leaves the 9.8m/s^2 throughout all of the examples so I thought that it stayed that way.

    You've just made my life so much easier.
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