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Homework Help: Confusing with Relative Motion

  1. Feb 28, 2010 #1
    1. The problem statement, all variables and given/known data
    An escalator is 20.0m long. If a person stands on the escalator, it takes 50.0s to ride from bottom to the top.

    a) if a person walks up the moving escalator with a speed of 0.500 m/s relative to the escalator how long does it take for the person to get to the top.

    2. Relevant equations

    V = d / t

    3. The attempt at a solution

    I'm not understanding relative motion too well. I begin by calculating the velocity of the escalator which is 0.400 m/s. Since the person is walking up with a speed of 0.500 m/s in relation to the escalator, does that mean that if the escalator was not there, he'd be walking at 0.500 m/s? Why would I not add the two velocities (.4 + .5) and isolate for the time?
  2. jcsd
  3. Feb 28, 2010 #2


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    It is better to think of this way: It means that if the escalator were not moving, he'd be walking at 0.500 m/s.
    Why do you think you should not do that?
  4. Feb 28, 2010 #3
    Not quite sure. I guess I thought that if it was moving relative, it would mean that it moves that speed and the speed of the elevator. Blech.

    Would that mean that anything with relative motion requires something to be stationary, such as in this question?
  5. Feb 28, 2010 #4


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    All motion is relative. You may think that you are at rest sitting in front of your computer reading this message, but
    relative to the center of the Earth you are moving at about 1,000 miles an hour
    relative to the Sun you are moving much faster
    relative to a galaxy far far away much much faster.

    Get the picture?
  6. Feb 28, 2010 #5
    Yeah, makes sense now. Thanks!
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