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Homework Help: Free fall two objects

  1. Mar 30, 2016 #1
    1. The problem statement, all variables and given/known data
    A stone is thrown vertically upward from the top of the tower with a velocity of 15m/s. Two seconds later a second stone is dropped from the top of the tower.if both the stone strike the ground simultaneously find the height of the tower

    2. Relevant equations

    3. The attempt at a solution
    I used two methods and got different answers which is confusing me

    METHOD 1:
    Xf1 = 15t-4.9t^2
    Xf2 = -4.9(t-2)^2
    Xf2 = -4.9t^2 +16.9t -16.9

    15t -4.9t^2 = -4.9t^2 +16.9t -16.9
    -1.9t = -16.9
    t= 8.89s
    h=253.90 m

    METHOD 2:
    Xf1= 15(2) -4.9(2)^2
    Xf1 = 10.4m
    Vf= Vi + at
    Vf= 15 -9.8(2)
    Vf = -4.6m/s
    Xf1= 10.4 - 4.6t -4.9t^2
    Xf2 = -4.9t^2

    10.4 -4.6t -4.6t^2 = -4.9t^2
    t = 2.26s
    h= 25.03m

    Which solution is right?Why am I getting different answers? Is it because the 15m/s velocity is upwards and will reach maximum height? Im really confused
  2. jcsd
  3. Mar 30, 2016 #2


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    Science Advisor
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    Gold Member

    I haven't check it all but....

    4.9 * 4 = 19.6 not 16.9
  4. Mar 30, 2016 #3
    Thank you
  5. Mar 30, 2016 #4
    Sometimes you get the hard things right but make mistakes on the easy things
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