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Homework Help: Simple kinematics

  1. Jan 20, 2010 #1
    1. The problem statement, all variables and given/known data
    At the airport, persons A and B walk at an average speed of 0.90 ms-1 and 0.80 m s-1 respectively. Person B now walks on a elevator but person A does not. The elevator moves at the speed of 0.50 m s-1. How much faster will person B take to travel 50 m compared to person A ?
    A. 125 s
    B. 44.4 s
    C. 17.1 s
    D. 26.8 s
    E. 77.7 s

    2. Relevant equations
    s = vt

    3. The attempt at a solution
    On the manual, it is written : New speed of person B = 0.80 + 0.50 = 1.3 m s-1

    I find it strange because B walks on elevator. Maybe it means travelator.
    But if it is really elevator, the speed of person B should be 0.5 ms-1 because he can't walk in elevator.

    For both cases, there are the answers on the choices. So what is the best interpretation for the question?

    *I feel so stupid asking this question...:frown:
  2. jcsd
  3. Jan 20, 2010 #2
    Maybe they do mean a travelator.
    A person can walk in an elevator. In that case his velocity will be a vector resultant.
    However the vector resultant speed comes out to be 0.9433 m/s which does not match any of the choices.
  4. Jan 20, 2010 #3
    Hi aim1732

    Yeah actually B can walk in elevator, although it is not common case to be considered. And if B really walks in elevator, I think we still consider the elevator's speed because it is in the same direction as the distance traveled.

  5. Jan 20, 2010 #4
    Can't say.
    The question says distance of 50m, so why talk about direction in the first place?
  6. Jan 20, 2010 #5
    Hi aim

    Hm...because I think B can't move in the direction of the resultant velocity. B can only move in the direction of 0.80 ms-1 or the elevator. But I just realized your point of view. You only consider the resultant velocity and the distance that needed to be taken.

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