1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    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


    User Avatar
    Homework Helper

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook