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Homework Help: Constant accleration problem

  1. Sep 13, 2011 #1
    1. The problem statement, all variables and given/known data
    I need help on this hw problem I cant seem to figure it out.
    A objects constant acceleration is north at 40 m/s^2. At time 0 its velocity vector is 15 m/s west at what time will the magnitude of the velocity be 30 m/s?

    2. Relevant equations

    This is the eq i used: delta T =(Vf-Vi)/a
    I set Ti as 0.
    3. The attempt at a solution
    So I got 30-15/40= .375 secs

    I also tried subtracting gravity from acceleration to give (30-15)/(40-9.8) =.496 secs

    These are apparently wrong. what should i be doing?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 14, 2011 #2
    there is direction involved here so you cannot just pluck values into a formula

    so the question nicely sets north (y-axis) and west (x-axis) for you.

    so question wants the time when the MAGNITUDE of velocity is 30m/s, which means your x and y components of the velocity, will have to give you a resultant of 30m/s

    notice your x-component velocity of 15m/s doesn't change since there is no acceleration in this direction, acceleration is only towards north (y-axis)

    so in order to find the y-component velocity THAT will give you 30m/s resultant velocity, you have to use Pythagorean theorem as it is a triangle. draw it out and you will see

    so its 302 = 152 + y2, where y is your velocity in y-axis direction

    so solving, y = 26m/s

    so now you use your formula ALONG THE Y-direction

    time taken = { vf,along y-axis - vi, along y-axis } / a along y-axis
    which is

    (26 - 0) / 40 = 0.65s , the initial velocity is 0 because the object is moving west, there is no y-direction velocity
  4. Sep 14, 2011 #3
    ahhh, so i basically need to formulate the directional components before i can solve for t
  5. Sep 15, 2011 #4
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