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Free body diagram of person in elevator! Help

  1. May 24, 2013 #1
    1. The problem statement, all variables and given/known data
    A man is on an elevator
    - Draw a FBD of the man if the elevator accelerates upwards
    - Draw an FBD of the man if ele. accelerates downwards


    2. Relevant equations

    F=m*a
    w=mg

    3. The attempt at a solution

    My FBD For the first one "elevator accelelartes upwards" means that the net force must point upwards, the only two forces acting are the normal force and the weight force. I know that when you go up the elevator you should feel heavier, which is measuring the magnitude of the normal force

    So the normal force will have a greater magnitude pointing "up" and the weight force will be a smaller arrow pointing down.. as so..


    /\
    |
    |
    | Normal force
    _
    |
    \/Weight force

    The second part will be opposite, so you will have a smaller normal force and greater weight force, and will feel lighter than usual because of a smaller normal force acting on you.

    First part given by n= may +w
    second n=-may +w

    Is this correct? is something wrong here?

    Thanks
     
  2. jcsd
  3. May 24, 2013 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Right.
    I would write both cases as ##F_N= ma_y +w## and let the acceleration be negative in the second case to keep the signs consistent.
     
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