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Homework Help: Relative Velocity Sidewalk

  1. Sep 4, 2013 #1
    1. The problem statement, all variables and given/known data

    A passenger rushing to catch a plane at the airport walks on a moving sidewalk at a speed of 3.0 km/h relative to the sidewalk in the direction that the sidewalk is moving. The sidewalk is 135 m long and moves with a steady velocity of 1.0 km/h.

    The question has 4 parts:
    1. 1. How long (in s) does it take for the passenger to get from one end of the sidewalk to the other, that i *to cover the 135 m? I found this to be 122s.
    2. How much time does the passenger save by taking the moving sidewalk instead of just walking beside it? Here, I found out by walking, it takes 162s to walk beside it, leaving a 40s difference in time.
    3. Through what distance does the passenger walk relative to the moving sidewalk? I haven't figured this one out becasue I have no idea what its even asking (yes, I know you're not supposed to say that but honestly, I don't understand what my professor is looking for).
    4. and 4, the one that I have an issue on. "If the passenger has a stride of 90cm, how many steps are taken in going from one end of the moving sidewalk to the other?"

    2. Relevant equations

    none that i can think of to apply

    3. The attempt at a solution
    Since I'm really posting on here for the last part, here's what I did.

    I converted 135 m (length of the sidewalk) to 13,500 cm, since the length of the stride is in cm. So then, I did 13,500cm/90cm and got 150. The computer program that my professor uses for homework said that was wrong. Then figuring that a "stride" was the length between two steps, so then I divided 150 by 2 (for 2 steps) and got 75, which was also wrong.

    If anyone wants to help on what the third part is even asking (like i said, I know you're supposed to attempt them on your own but I seriously do not even understand what it is asking.)​
  2. jcsd
  3. Sep 4, 2013 #2
    Here are my thoughts:
    First figure out what is the time between steps. Then what is the stride, relative to the ground, considering how fast the person and sidewalk is moving. Obviously the stride will be longer. Now divide the length of the sidewalk by the length of the stride.
  4. Sep 4, 2013 #3


    User Avatar

    Staff: Mentor

    The key to part 4 is part 3 :smile:

    The passenger is walking along a surface. That surface happens to moving with respect to something else (the ground), but ignore that. Concentrate on the person walking and the surface he's walking on as if they were the only thing in existence. You've figured out the time he spends walking on the surface in (1). You're given the speed with which he walks on the surface. So what distance does he walk on the surface?

    Then for (4), if you know the distance walked from (3) and the stride length, how many strides make that distance?
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