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Homework Help: Finding 2D coordinates in different frames

  1. Oct 16, 2012 #1
    Hi guys, i need help for homework, it seems easy, but i can't do it:cry:, no calculation to do only writing 2D coordinates in different frames.

    1. The problem statement, all variables and given/known data

    The hallmark of an inertial frame is that any object which is subject to zero net force will travel in a straight line in a constant speed. to illustrate this, consider the following: I am standing on a level floor at the origin of an inertial frame S and kick a frictionless puck due north across the floor. (a) write down the x and y coordinates of the puck as functions of time as seen from my inertial frame. (Use x and y pointing east and north respectively). Now consider two more observers, the first at rest in a frame S' that travels with constant velocity v due to east relative to S, the second at rest in a frame S'' that travels with constant acceleration due east relative to S at that same moment). (b) Find the coordinates x' and y' of the puck and describe the puck's path as seen from S'. (c) Do the same for S''. Which of the frames is inertial?

    2. The attempt at a solution

    for the frame S : x(t) = 0 and y(t) = v(0).t

    for the frame S' : looks like a translation from S
  2. jcsd
  3. Oct 16, 2012 #2

    Simon Bridge

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    Welcome to PF;
    Do you think you'd be expected to use Lorentz transformations or is classical relativity OK (would you get the same answer?)

    Which frames do you think are inertial?
  4. Oct 16, 2012 #3
    I am not expected to use neither Lorentz transformations nor classical relativity, this is a classical mechanics problem. I think S and S' are inertial and S'' is not, because i think the first law of Newton is only true in S and S'.
  5. Oct 16, 2012 #4
    The time in classical mechanics is absolute
  6. Oct 17, 2012 #5

    Simon Bridge

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    You have to use one or the other.
    Classical relativity it is then.
    It is a matter of working out the v(t) in each frame ... you can do that by breaking the motion into components and adding/subtracting vectors.
    OK - you have your head on right then. The trick to to show this mathematically.

    You got frame S OK - only I'd call the speed of the puck u rather than v(0) to avoid confusion with the other frames (unless you happen to know that u=v).

    Frame S' has a constant relative velocity v perpendicular to the motion of the puck... so an observer stationary in S' will see the puck go in both x and y directions.

    S'' is just the same as S' except there is a constant acceleration involved.
    You know a kinematic equation for this.

    The real trick is relating these descriptions to the "Hallmark of an inertial frame".
    It is seldom good enough to write down the correct answers, you have to show the marker that you got those answers using sound reasoning in order to pick up all the marks.

    Trick: Your final answer should include the words of the question.
  7. Oct 22, 2012 #6
    Thank you Simon Bridge you've been very helpful.
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