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Finding acceleration of elevator using mass

  1. Nov 6, 2008 #1
    1. The problem statement, all variables and given/known data
    one day you take your bathroom scale with you into an elevator. the scale reads 150 lbs as the door closes. the reading varies between 120 lbs and 170 lbs as the elevator travels up. what is the acceleration of the elevator while its accelerating and decelerating?


    2. Relevant equations
    dynamics equations


    3. The attempt at a solution
    from the first part we know that the scale is telling us the normal force.Fg=Fn Fn=150lbs
    for the accelerating phase F= Fn-Fg, ma=170lbs - mg
    this is where i get stuck. where should i go from here?
     
  2. jcsd
  3. Nov 6, 2008 #2

    Doc Al

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    Staff: Mentor

    The scale always gives the normal force. The initial reading tells you your weight (mg).
    You're doing fine. Substitute the value of mg (the weight) and solve for "a". (Figure out your mass.)
     
  4. Nov 6, 2008 #3
    Fn = mg (for the first part) m= Fn/ g is that the mass? the units dont really match though

    and what do i substitute in for mg ? 150 lbs?
     
  5. Nov 6, 2008 #4

    Doc Al

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    Staff: Mentor

    Correct.
    It's up to you to use consistent units. What's the value of g in the English system? What's the unit of mass?
    Yes.
     
  6. Nov 6, 2008 #5
    thanks for the help
     
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