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Homework Help: Elevator Problem w/ app. Weight

  1. Sep 23, 2008 #1
    1. The problem statement, all variables and given/known data

    Henry gets into an elevator on the 50th floor of a building and it begins moving at t = 0 s. The figure shows his apparent weight over the next 12 s where w1 = 520, w2 = 720, and w3 = 920.
    The graph:
    Begins at t=0 at w1 and goes in a horizontal straight line until 2. At t =2 the line goes straight up to w2 and stays horizontal on w2 until t=10. At t=10 the line goes straight up to w3 and stays horizontal on w3 until t=12. At t=12 it goes back down to w2. That is the end of the graph.

    1. Is the elevator's initial direction up or down and how?
    2. What is Henry's mass? _____kg
    3. How far did Henry travel at t=12 sec?_____m

    2. Relevant equations

    F = ma
    Wapp = m(g + ay)
    Fsp = w + may
    X = Vot + .5a(t)^2

    3. The attempt at a solution

    Number 1- I got that it is going down but I don't understand why. I think it has something to do with the acceleration being positive. What is a clear explanation for why its inital direction is down?

    Number 2- I've been trying to use the apparent weight formula to figure out the mass but I can't do that without the acceleration. After working the equations out using the Wapp and the g, I got that " m = (520 / (9.8 + ay)) " But, I can't fully solve for m without the acceleration and I can't solve for acceleration without the total force or m. I seem to be in a hole.

    Number 3- Well, after I get the acceleration I can use time and Vo (which would equal 0, right?) to get the distance traveled.

    Thanks in advance for your help!
  2. jcsd
  3. Sep 24, 2008 #2
    For number 1, it's actually going up. Picture the elevator in your mind. If it's going up, then it will be pushing up against you harder than usual right?

    For number 2, all you need to solve for his mass is his "true weight". This is given to you in the problem if you look closely. F = ma.
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