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Homework Help: Help with a mechanics problem from Kleppner's book

  1. Mar 3, 2016 #1
    1. The problem statement, all variables and given/known data
    An elevator departs the ground at t=0 With uniform velocity. At T1 a boy drops a marbel through the floor of the lift that falls under constant acceleration g=9.8 ms^-2. If it takes marbel T2 time to fall down, what was the height at the T1.
    2. Relevant equations

    h= vT2 -.5g(T2)^2
    3. The attempt at a solution

    I simply replaced v in second equation by h/T1. And i got the h in terms of T1, T2. But the problem is, there is a clue given with this problem in the book. That is when T1=T2=4 h=39.2 now, in my equation, T1=T2 gives a height 0.
  2. jcsd
  3. Mar 3, 2016 #2


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    Science Advisor
    Homework Helper
    Gold Member

    you have to relook your 2nd equation for proper signage.
    the height that the elevator rises at T1 is the same as the height the marble falls at T2. The sum total of those 2 distances is 0 (h - h =0). Check signage.
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