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Word Problems

  1. Jul 22, 2009 #1
    1. The problem statement, all variables and given/known data

    A person that has a weight, on land, of 145 lb gets on an elevator to records an apparent weight of 122 lbs. What is the direction and magnitude of the elevator's acceleration?

    2. Relevant equations

    F=ma or F=mW

    3. The attempt at a solution

    I have no Clue how to do this any help will do :)
  2. jcsd
  3. Jul 22, 2009 #2


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    Staff: Mentor

    First, is the elevator accelerating upwards or downwards? How can you tell?

    What causes the change in apparent weight?
  4. Jul 22, 2009 #3


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    Gold Member

    Weight is a force, not an acceleration. Your weight is equal to F = ma = mg when standing on the Earth where g = -9.8m/s^2. When you're in an elevator, assuming you're being weighed, the acceleration of the elevator will either help cancel out or increase your weight which is something you can tell by our everyday experience. Since your mass doesn't change, you can determine the change in the acceleration you feel by comparing your weight on land and with the elevator in use. This change is equal to the elevator's acceleration.
  5. Jul 22, 2009 #4
    Im still Confused . I know that the the elevator is accelerating downwards, reducing his apparent weight but how do I solve for the Magnitude?
  6. Jul 22, 2009 #5


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    Gold Member

    Well you know the change in weight. With this, you can determine the acceleration since you can determine the mass from the initial part of the problem saying that someone weighs 145lb on Earth outside of the elevator.
  7. Jul 22, 2009 #6
    If he is accelerating downwards then there has to be a net force going down also. I changed to Newtons so 642.25-542.9=ma. m=65.53. You can solve for a. It is easier to visualize him being on a string. That is another way to measure weight. So that makes mg-T=ma. T is the weight shown then.
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