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Velocity and acceleration problem

  1. Feb 1, 2010 #1
    1. The problem statement, all variables and given/known data
    the position vecter r of a particle at a time t is

    r = icos t + jsin t +kt show that the magnitudes of the velocity and acceleration vectors are constant. Describe the motion

    3. The attempt at a solution

    velocity = i(-sint) + j(cost) +k

    magnitude is [tex]\sqrt{(-sint)^2 + (cost)^2 + 1}[/tex] = [tex]\sqrt{2}[/tex]

    acceleration = i(-cos t) - j(sin t)

    magnitude is [tex]\sqrt{(-sint)^2 + (-cost)^2 }[/tex] = [tex]\sqrt{1}[/tex] = 1

    but im not sure how to discribe the motion, at first i was thinking of an object moving at a constant velocity of [tex]\sqrt{2}[/tex] and to move at a constant velocity it acceleration isn't changing so should that not mean the acceleration is 0 instead of 1?
     
  2. jcsd
  3. Feb 1, 2010 #2

    vela

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    You've shown that the particle has a constant speed, not a constant velocity. While the particle moves with constant speed, the direction it's moving continuously changes.

    Ignore the z-component for right now. If x=cos t and y=sin t, what path would the particle follow in the xy plane?
     
  4. Feb 1, 2010 #3
    im trying to think about it, it follows a wave, up and down? but can it follow a sin and cos wave at the same time? do they not start from different points? i'm a bit confused
     
  5. Feb 1, 2010 #4

    vela

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    If you're given a value of t, you can calculate what x and y equal and plot that point on the xy-plane. Try doing that for, say, t=0, pi/6, pi/4, pi/3, pi/2, 2pi/3, 3pi/4, 5pi/6, pi.
     
  6. Feb 1, 2010 #5
    thanks a million for the replies.

    After plotting it out, i think its moving at constant speed,not accelerating around the axis in a circular motion of radius 1?
     
  7. Feb 1, 2010 #6

    vela

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    It's moving with constant speed, but it's accelerating because the direction of the velocity is changing with time.

    So now add the motion in the z direction back in. So while the x and y coordinates trace out a circle, the z coordinate increases in time at a constant rate. How would you describe that path?
     
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