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Homework Help: Two objects in free fall

  1. Mar 30, 2016 #1
    1. The problem statement, all variables and given/known data

    A stone is dropped from rest from the roof of a 100-m high building; 2.00 s after that, a second stone is thrown straight down with initial speed v0, and the second stone passes the first stone 21.6 m above ground. (a) When are the stones at the same height? (b) What was the initial speed, v0, of the second stone, in m/s? (c) What are the speeds of the two stones, in m/s, just before they hit the ground?

    h= 100m
    h(meet)= 26.1m
    V1i= 0m/s
    2. Relevant equations
    Xf=Xi +Vt +0.5at^2

    3. The attempt at a solution
    (a). Xf1= -4.9t^2
    Xf2 = Vo(t-2) -4.9(t-2)^2
    -73.1 = -4.9t^2

    (b). -73.1 = Vo(1.86) -4.9(1.86)^2
    Vo = 30.19 m/s

    (c). Vf = Vi + at
    Vf = 0 -9.8(3.86)
    Vf = 37.83 m/s

    Vf= 30.19 -9.8(1.86)
    Vf= 48.42m/s

    Am I right? Not sure if I am
  2. jcsd
  3. Mar 30, 2016 #2


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    You don't give the time, although the next entry seems to have used the correct time.

    You calculated the speed of the both stones at 21.6 m above the ground.
    Last edited: Mar 30, 2016
  4. Mar 30, 2016 #3
    I could be wrong, but I found slightly different values. For time, t = 4s, and initial speed V0 = 29,4. Are you sure about your results?
  5. Mar 30, 2016 #4


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    I think you're right.

    I didn't even think to check the distances OP came up with.

    100 - 21.6 = 78.4, not 73.1

    If it's supposed to be 26.1,
    then 100 - 26.1 = 73.9, not 73.1
  6. Mar 30, 2016 #5


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    Something isn't adding up here:
  7. Mar 30, 2016 #6
    @Ab17 double check your computation in (a).
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