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Need help with vectors and avg velocity

  1. Oct 25, 2012 #1
    1. A car with an initial direction of north moves clockwise one quarter a circle of radius 30 m in 15 s at a constant speed. What is the average velocity?

    a) 1 m/s north
    b) 1.41 m/s northeast
    c) 1.57 m/s southwest
    d) 2.83 m/s northeast

    displacement/total time = avg velocity

    15pi/15s = pi = 3.14 m/s

    but none of the answer choice are 3.14 m/s. also how do you find the direction?

    2. The velocity of a jet with respect to the ground is 200 m/s at an angle of 30 degrees north of east. The velocity of the jet with respect to the air is 150 m/s at an angle of 60 degrees north of east. What is the velocity of the air with respect to the ground?

    a) 102 m/s 17 degrees south of east
    b) 134 m/s 49 degrees south of east
    c) 134 m/s at 17 degrees north of east
    d) 203 m/s at 65 degrees north of east
    e) 112 m/s at 50 degrees north of east


    I the jets velocity into its components and got vy = 100 and vx = 173. Then with respect to the air vy = 130 vx = 75.

    Then i took the sqrt of (173 - 130)^2 + (100 - 75) ^ to get 103 m/s but I think im doing something wrong.
     
  2. jcsd
  3. Oct 26, 2012 #2

    haruspex

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    Velocity is a vector. It's the change in the position vector divided by the time.
    What is the change in the position vector?
    Yes, you're mixing up the vx's with the vy's. Try again, making sure you only take differences of speeds in the same direction.
     
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