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Homework Help: Elevator force problem

  1. Oct 4, 2008 #1
    1. The problem statement, all variables and given/known data

    A person with a weight of 500N stands in an elevator on a scale.
    from 0-5s his weight reads 500N.
    from 5-10s his weight reads 700N.
    from 10-15s his weight reads 500N.
    from 15-20s his weight reads 300N.

    Find his acceleration (in each time interval) and his velocity at 5s, 10s, 15s, and 20s.

    3. The attempt at a solution

    I think my accelerations are correct:
    (Fn-mg) /m=a

    0-5s: (500N-500N) / 51kg= 0m/s^2
    5-10: (700-500)/51=3.92
    10-15: (500-500)/51=0
    15-20: 300-500/51= -3.92

    Now, I think my velocities seem a little unreasonable.

    at 5: v=0
    10: v=0m/s + (3.92)(10)=39.2m/s <- that is really fast!
    15: v=39.2m/s + (0)(15)= 39.2m/s
    20: 39.2m/s+(-3.92)(20)= -39.2m/s

    thanks for any help
  2. jcsd
  3. Oct 4, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    That time is the time in the interval for which the acceleration is "a", not the time from the beginning. I'd write it as: Vf = Vi + aΔt.
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