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Homework Help: What am I doing wrong? Simple Projectile motion calculations.

  1. Nov 4, 2014 #1
    Ok, I have used every calculator I can find, typed it in several different ways for the past three hours and at this point I am giving up and asking for help.

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

    A rock is thrown from the edge of a 14 meter high cliff with a velocity of total magnitude of 7m/s. Find the final velocity with which the rock hits the ground if it was launched initially upward.

    2. Relevant equations
    OK so we have
    delta D - 14m (downward, negative)
    V1 - 7m/s
    V2 - Solve for
    A - 9.8m/s^2 (down) (let upward be positive so this would be negative)
    delta T - Solve for.

    For equations - (upward throw)
    Vfinal^2 = Vinitial^2+2AD
    Vfinal^2 = 7^2 + 2 (-9.8^2)(-14)

    So V final = square root ( (7^2) + 2(9.8^2)(-14)
    Final answer I got is 52.32.

    This is a practice question so they list the answer and the answer is supposedly plus or minus 17.98m/s.

    What am I doing wrong???!?!?

    I thought maybe calculator when squaring a negative number does not turn it into a positive (ie. -9.8x-9.8 = positive number) and therefore threw off answer but when done that way it is multiplied with negative 17 and I cannot ask for square root of a negative..

    Been trying to troubleshoot this too long and its driving me nuts! Any and all help is SO appreciated.
  2. jcsd
  3. Nov 4, 2014 #2


    User Avatar

    Staff: Mentor

    Hi dylanjames. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Get rid of that ^2 around acceleration, it doesn't belong.
    Last edited by a moderator: May 7, 2017
  4. Nov 4, 2014 #3
    ........thank you. boy doesn't that make me feel stupid!!!!!
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