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Homework Help: Elevators and Acceleration

  1. May 24, 2004 #1
    This should be a simple problem, but I think I'm making it harder than it is.

    In this problem, I am interested in maximum acceleration for an elevator during normal operation. While the elevator is at rest, on the ground floor, I get in, put down my bathroom scale and stand on it. I continue standing while the elevator is going up. During my trip to the 45th floor, the scale reading increases by a max of 25lbs.

    Assumptions:
    It doesn't matter what floor you go to.
    When the elevator accelerated upward, the apparent weight is greater than mg by the amount ma. It's as if gravity were increased from g to g+a.

    According to an equation I found in my textbook...
    Fn-mg=ma (where a is in the y direction)
    Fn=mg+ma (where Fn is the reading on the scale, the apparent weight)

    Since the reading is given in lbs, I am going to use 32.2ft/s^2 for gravity.
    I am going to start with an arbitrary weight of 100lbs, which would give a max weight of 125lbs.
    And W=mg so....100lbs=(m)(32.2ft/s^2)=3.11slugs

    Fn=mg+ma
    125lbs=(3.11slugs)(32.2ft/s^2)+(3.11slugs)(a ft/s^2)
    125lbs-100.14lbs=(3.11slugs)(a ft/s^2)
    24.86lbs=(3.11slugs)(a ft/s^2)
    7.99ft/s^2=a

    If I did the problem right...
    Is 7.99ft/s^2 my answer for max acceleration? Or do i add that to 32.2ft/s^2 for a max acceleration of 40.19ft/s^2?

    Thanks! Kelli :yuck:
     
  2. jcsd
  3. May 24, 2004 #2

    Doc Al

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    Staff: Mentor

    You did the problem correctly. (But see my steps below.) You calculated the max acceleration (assuming a 100 lb weight). Why would you add it to g? (g is the acceleration of a freely falling body--the elevator isn't falling, is it?)

    Here's how I'd do it:

    Fn - mg = ma
    (mg + 25) - mg = ma
    25 = ma

    a = 25/m = 25 (32.2/100) = 8.05 ft/s^2 (upwards)
     
  4. May 25, 2004 #3
    Do I need to even assume a 100lb weight? What if i used a weight different than 100lbs, wouldn't that affect the acceleration?
     
  5. May 25, 2004 #4

    Doc Al

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    Staff: Mentor

    Yes it would. The same net force (25 lbs) acting on a different mass would yield a different acceleration.
     
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