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Homework Help: Bolt in a ceiling problem

  1. Apr 26, 2013 #1
    1. The problem statement, all variables and given/known data

    It's a question from IE Irodov's General Physics -->

    An elevator car whose floor to ceiling is equal to 2.7m starts ascending with constant acceleration 1.2m/s2. 2s after the start a bolt begins to fall from the ceiling of the car .Find

    (a) the time after which bolt hits the floor of the elevator.

    It's only the first part of the problem ...

    2. Relevant equations

    3. The attempt at a solution

    After two seconds of start, the elevator catches a speed of v = 2.4 m/s ...
    Then, I assumed that after t seconds the event of bolt falling on floor occurs ...

    So the elevator floor covers a distance of 2.4t + 1/2at2
    And the bolt covers 1/2gt2

    Using some intution, I got

    2.4t + 1/2at2 + 1/2gt2 = 2.7
    Solving it, my answer comes out t = 0.5 seconds but the answer is 0.7.

    Please don't solve it using relative speed of bolt with respect to elevator or any other method... just let me know what's wrong with my solution.
  2. jcsd
  3. Apr 26, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor


    You neglected the initial speed of the bolt. This assumes it's dropped from rest.

    Correct that and you'll get the right answer.
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