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Question regarding velocity

  1. Jun 16, 2003 #1
    if you were to look at a "freeze frame" of time (i.e. a movie that's been paused)

    and you saw a baseball in the air. how would you know that it had a velocity??

    i mean, a baseball is a collection of atoms. how would you know, just by observing it, that it has a velocity (and what velocity? can you tell?)

    At each moment in time, each object must have an "instantaneous velocity". How do you explain that in terms of the atomic scale??
  2. jcsd
  3. Jun 16, 2003 #2
    you wouldn't. unless somehow you were able to measure the objects energy level (mass) and knew beforehand its energy (mass). it would be completly immpossible given our current technology becuase the change in mass (energy) would be so small.
  4. Jun 17, 2003 #3
    As maximus said, you would not be able.
    (techniclally, each frame of the moive took about 1/24 of a second, so there is what is called motion blur that allows you to know an approximate value of the velocity, but i am sure this is not what you are looking for).

    (I don't really understand what do u mean by "in term of atomic scale" here).
    Well, we also cannot know the instantaneous velocity from only a single snap a the object in a certain moment, in order to know the instantaneour velocity we normally need to have a graph of displacement against time, if we only have a single point of the graph (ie, a single snap about the object), we will not be able to calculate its instantaneous velocity.
    What we do to calculate the instantaneous velocity is that we study the behaviour of the graph near the point of interest using calculus, then from the info we get about the object just before and just after the point of interest, we can know its instantaneous velocity.
    You can think of it (although this way is not perfectly right, but for the moment it might help) as calculating the velocity on a very small interval, say 10-1000 seconds, you will get a value of the velocity that you can almost call instantaneous (but it is not, remember, to calculate the instantaneous velocity you need to use calculus).
    If you already know some calculus, let us help you know how to use it to find instantaneous velocties.

    I hope i helped :smile:.

    edited for formatting mistake.
    Last edited: Jun 17, 2003
  5. Jun 17, 2003 #4


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    Hope this is not off topic, but what you have described happens to be my favorite illustration to explain the Heisenberg Uncertainty Principle to lay people. What you're describing is like a photograph taken with a camera that has an infinitely fast shutter speed such that, during the exposure of the film, no time at all passes. If someone were to show you such a photograph and ask you the question, "where was the ball located when this picture was taken?", you could give them a very precise answer. There would be no blurring at the edges of the ball's image, and you could tell with great precision where the ball is and where it is not. However, if the same person ask you what direction the ball was traveling when the photo was taken, you would not have a clue.

    However, if you use a slightly slower shutter speed and a longer exposure of film, the ball in flight would be "blurred". In this photo, you could have some idea of the direction of travel of the ball. If you go to a very long exposure, such that the shutter was open throughout the entire flight of the ball, the resulting image of the ball would be one long blurr. In this photo, you could give a highly detailed account of where the ball travelled, but if someone were to ask you where the ball was located when the photo was taken, your answer would be the entire area covered by that blurr.

    IOW, the more you know about one aspect (location, in this case), the less you will know about a concurrent aspect (motion).

    It is an in precise analogy, but at least it makes the nature of the Principle comprehensible to most people.
    Last edited: Jun 18, 2003
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