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Quick question

  1. Nov 16, 2011 #1
    I would like to know if this is possible or not.
    If two people, A and B running side by side are moving at the same exact speed, then to a third person watching them, it looks like indeed they are running the same speed. But is it possible from A's point of view, that B looks to be moving at a faster speed compared to A himself.

  2. jcsd
  3. Nov 16, 2011 #2


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    Sure. If A is rotating while moving, it will briefly appear as if B is gaining on him (or falling behind). Otherwise, no.

    Some riffs on a theme: it is possible that C could see them as if they're moving at the same speed, even when they're not (A and B are running in a circle around C.) But this cannot be so if, as you say, A and B are indeed "moving at the exact same speed".

    If this is a test of a real-world scenario, you should probably give us the whole story, as there will surely be other factors coming into play.
  4. Nov 16, 2011 #3
    This was just inspired by some thoughts I had after my math teacher went off on some physics talk.

    So, what if A and B are both running in a straight line, no circular motion for either one?

    For instance, imagine an average sized person, A, say 6 feet tall, running at 10 miles per hour. Alongside him is a small person, B, 3 feet tall, also running 10 miles per hour.

    Lets say for B to travel any distance that A goes, he must take 2 steps to equal 1 step of A.

    As they run next to each other, A looks over noting that B is keeping pace thus obviously running the same speed. But Since 2 steps of B's is only equal to 1 of A, then to A, B seems to have to move twice as fast as what A has to move just to be running at the same speed.

    So from A's point of view wouldn't it seem like the ratio of how fast he is moving proportional to his body is half that of how fast B is moving propitional to B's body?
  5. Nov 16, 2011 #4


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    Surely the test of whether one appears to be moving at a different speed from the other is ... well ... observing whether one is moving away from the other.
  6. Nov 16, 2011 #5
    I could very well be thinking about this in just a stupid way, I know. But lemme put it this way.

    If a massive giant (yes, they now exist) walks a lesisurely pace of (I'll just use the same measurements as before) 10 miles an hour. A person 6 feet tall also moves 10 miles per hour. However this person has to run to get to that speed while the giant just needs to walk. For the giant it doesn't seem like he is moving fast at all, because 10mph is just his usual unit speed when he walks. But When he looks down next to the average person next to him, that person has to move quite fast for what he is used to. So to the giant, it looks like the person is moving faster then what he has to move.

    Basically if I can cover a certain distance while just walking and another person covers that same distance in the same amount of time that I do, but has to run, then the runner looks like he moves his body faster then what the walker has to move his body?

    I might just be making ridiculous notions, Idk. If so, I apologize.
  7. Nov 16, 2011 #6


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    OK so we're talking about perception.

    Sure it's possible that you perceive it that way momentarily. But I imagine that would last a fraction of a moment until you saw that they weren't receding from you.
  8. Nov 16, 2011 #7


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    To quote one of our more knowledgeable forum members, "Surely the test of whether one appears to be moving at a different speed from the other is ... well ... observing whether one is moving away from the other."
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