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Homework Help: Average acceleration using vectors please help

  1. Sep 17, 2007 #1
    At one instant a bicyclist is 30 m due east of a park's flagpole, going due south with a speed of 18 m/s. Then, 14 s later, the cyclist is 45 m due north of the flagpole, going due east with a speed of 9 m/s. For the cyclist in this 14 s interval, find each of the following.
    A)average acceleration

    What i did was, i used the fact that a=v/t. So i pluged in a=(9+(-18))/14. The acceleration I got was 1.93. that is the wrong answer. Can someone please tell what i did wrong and help me figure out a way to get the angle. thank You
     
  2. jcsd
  3. Sep 17, 2007 #2

    learningphysics

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    velocity and acceleration are vectors... can you write the two velocities in vector form... (use i and j vectors). Then calculate average acceleration just as you did, but you'll be subtracting two vectors...
     
  4. Sep 17, 2007 #3
    sorry i missed type what i did i did subtract i did a=9-(-18)/14 and i got 1.93. i know the answer is 1.37 but i dont know how to get it
     
  5. Sep 17, 2007 #4

    learningphysics

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    You can't subtract like that because east and south aren't along the same line... write the two velocities as vectors (using i for east west... j for north south).

    For example: 20m/s west is

    [tex]\vec{v} = -20\vec{i}[/tex]

    20m/s north is:
    [tex]\vec{v} = 20\vec{j}[/tex]

    Does this make sense?

    If the notation doesn't make sense... then try it like this... what is the average acceleration in the east west direction (taking east as positive) ?

    what is the average acceleration in the north south direction (taking north as positive) ?

    What is the magnitude of the net acceleration?
     
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