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Difficult Free Fall/Kinematic Problem

  1. Jul 27, 2007 #1
    1. The problem statement, all variables and given/known data

    A first stone is dropped from the roof of a building. 2.40 s after that, a second stone is thrown straight down with an initial speed of 33.0 m/s, and it is observed that the two stones land at the same time.

    (a) How long did it take the first stone to reach the ground?
    _____________ s
    (b) How high is the building?
    _____________ m
    (c) What are the speeds of the two stones just before they hit the ground?
    _____________ m/s (first stone)
    _____________ m/s (second stone)

    2. Relevant equations

    Free Fall Equations:
    Vf = g*t
    d= .5*g*t^2

    Kinematic Equations

    3. The attempt at a solution

    If I knew any variables, I might be able to use the Free Fall Equations, but no variables are given for the first stone. The second stone only tells me Vi and a little bit of information about t..but not enough to get me started really. I'm truly lost on this one. I'm sorry my attempt at a solution seems pethedic! :frown:
  2. jcsd
  3. Jul 27, 2007 #2
    first, you have to use all the values they give you. If you let t_1 represent the time for the 1st stone then t_2, the time for the 2nd stone, is equal to t_1 - 2.4sec. Can you figure out what the values for the d's should be? or would it be easier to relate the two d's like how I related the 2 t's. Also, can you figure out what the value for the acceleration should be?
  4. Jul 27, 2007 #3
    The distances for both stones are the same since they're being dropped from the same height..I tried setting the equations equal to each other but it said my answer was wrong.

    Would the acceleration for the first stone be 9.8 m/s2? For both? I'm not sure my answer keeps on being wrong.
  5. Jul 27, 2007 #4


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    Homework Helper

    Write d=vt+1/2 at^2 for both stones.
    You know that d is the same
    You know v for both stones
    You know a. Remember that although one stone was thrown, as soon as it left your hand it isn't being accelarated by you anymore, only by gravity - exactly the same as droping it.

    You will get an eqution with 2 unknown times, but you know how the times are related T1=T2+2.4 so you have only one unknown, you can solve for T and then use the same equation to find d.
  6. Jul 28, 2007 #5
    Thanks! It took me awhile, (made many silly little mistakes), but I finally got it! =)
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