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What do physicists refer to when they say a object is at a certain speed?

  1. Aug 1, 2012 #1
    [STRIKE]Physicians [/STRIKE]Physicists oftenly state that it makes no sense to say that a certain object is moving at a certain speed if you do not have a reference point, but they do this very often. What is their referential point when they do not specify it?
    Last edited by a moderator: Aug 1, 2012
  2. jcsd
  3. Aug 1, 2012 #2
    Re: What do physicians refer to when they say a object is at a certain speed?

    But when physicists say it, they assume that you are observing from a well defined inertial (usually, but in certain cases maybe not) reference frame. If it is just a general statement, you just assume there is a frame of reference that you are observing from and in that frame of reference, the object is moving at that velocity.

    If you have a specific question, there may be an assumed frame of reference. If I say that I can bike 25mph I mean with respect to a person standing on the ground which can, for most purposes, be approximated by an inertial reference frame. Other situations may have different assume frames.
  4. Aug 1, 2012 #3
    DrewD is right, if they do not say "moving at __ m/s relative to _______" they probably mean the ground of the earth.
  5. Aug 1, 2012 #4
    In most cases, speed is described relative to the earth's surface. For most applications of classical mechanics, this works fine, but if you examine things more fundamentally, you find that this description of speed isn't perfect. The idea that you can only describe speed relative to another object is fundamental to the theory of special relativity. A common description of this idea goes kind of like this:

    You're floating through deep space at an unknown speed, with nothing in sight but distant stars. In the distance, you see another person come into view, getting closer to you. If you try to answer the question: "how fast is the person traveling?" you'll find you have a hard time doing this. Maybe you're perfectly stationary, in which case, you could measure their speed rather simply. However, if you don't know how fast you're traveling, maybe the other person is stationary and it's you who is flying through space; or you could both be moving. From all we know about physics today, it seems to be impossible to find an answer for this.

    Therefore, Einstein concluded that the only way to describe speed is relative to another object. So far as we know, the only "speed" that the person floating through space has, is a speed relative to you (or relative to another reference point). There's no way to tell whether one of you is "stationary."

    For things happening on earth, we've always used the earth's surface as a reference point, and this becomes second nature. However, if you fly away from the earth in a space ship, how do you measure your speed? If you measure how fast you're going relative to the earth, you get a different speed relative to mars, or relative to the sun, or whatever you compare it to. Just the same, your speed relative to the sun is different than your speed relative to a neighboring star, or the center of the galaxy. There's no definitive reference point that allows you to declare an absolute speed.
    Last edited: Aug 1, 2012
  6. Aug 2, 2012 #5
    Speed reletive to the CMB is probably the most definitive. An observer who can look in any dirrection and see the same CMB spectrum would be at rest reletive to the CMB. If they were not at rest, the CMB would be red-shifted in one dirrection and blue-shifted in the opposite dirrection.

    It should be noted that 2 observers who are at rest with respect to the CMB will not be at rest with respect to each other. Due to cosmic expansion the distance between them will increase by approximately 1/130 of 1% per million years.

    (CMB = Cosmic Microwave Background radiation)
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