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Simple kinematic problem

  1. Nov 21, 2011 #1
    1. The problem statement, all variables and given/known data

    An airplane maintains a constant acceleration of 4.0m/s^2 as it speeds up from 16m/s [E] to 28 m/s [E].

    A what is the average velocity?

    b) how long does the airplane accelerate?

    c) when is the instantaneous velocity equal to the average velocity for the interval?

    a= 4.0 m/s^2

    vi= 16m/s [E]
    vf= 28m/s[E]


    2. Relevant equations



    3. The attempt at a solution

    i found the answer to A and B

    Average velocity = 22 m/s. Total time = 3.0 s

    I don't know how instantaneous velocity is related to average velocity for the interval. Or how to find the answer. Someone help me?
     
    Last edited: Nov 21, 2011
  2. jcsd
  3. Nov 21, 2011 #2
    If you found the average velocity to be 22m/s it is asking when the instantaneous velocity is 22m/s which should just be V = vi + at
     
  4. Nov 21, 2011 #3
    Ok got it thanks :)
     
  5. Nov 21, 2011 #4

    PeterO

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    Homework Helper

    When are the two numerically equal?

    Suppose you accelerated away from traffic lights for 10 seconds. After 10 seconds you may have reached quite a high speed, but your average speed will be less than that. At some time during the 10 seconds, the speedo will have shown a speed that just happens to equal your average speed [you won't have realised at the time, since you need the full trip to calculate your average speed]. When was that time?
     
  6. Nov 21, 2011 #5
    ahh ok thank you so much!
     
  7. Nov 21, 2011 #6

    PeterO

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    btw; Part C of this problem is a learning exercise, so that in future you will know the bit I highlighted red.
     
  8. Nov 21, 2011 #7
    ok thank you
     
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