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Average acceleration problem having trouble

  1. Sep 9, 2012 #1
    At a certain time a particle had a speed of 11 m/s in the positive x direction, and 1.3 s later its speed was 78 m/s in the opposite direction. What was the average acceleration of the particle during this 1.3 s interval?


    2. Relevant equations
    I have Vi=11m/s, Vf=78m/s and t=1.3 m/s


    3. The attempt at a solution
    My attempt at the solution has been the formula of (78-11)/1.3. This has gotten me the answer of 51.54 and I at am a real loss. I have read my book over and over and I juts need some guidance.
     
  2. jcsd
  3. Sep 9, 2012 #2
    At a certain time a particle had a speed of 11 m/s in the positive x direction, and 1.3 s later its speed was 78 m/s in the opposite direction. What was the average acceleration of the particle during this 1.3 s interval?

    a=(vi+vf)/t

    a=(11-78)/1.3

    a= -51.54 m/s2

    Your vf should have been -78 m/s since it was moving in the opposite direction.
     
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