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Can't Identify Error in Calculation

  1. Oct 28, 2016 #1

    RBF

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    [Mentor's note: This thread was moved to the homework section from General Physics, so it doesn't use the template.]

    Going over a basic kinematics problem with the prompt stating vi=0, x=1000m and Δt=5 and vf and a need to be solved using average v and a. Calculated vf=200ms and a=40m/s/s. But then I also calculated for the same variables using x=.5(Δv)t and all turned wonky and I can't figure out why. Solving for vf=(2)(1000m)(1/5) I get 400m/s. Any insight into what error(s) Iam making?
     
    Last edited by a moderator: Oct 28, 2016
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  3. Oct 28, 2016 #2

    phyzguy

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    Your first calculation looks to be wrong. Show more work on how you got to vf=200 m/s and a = 40m/s^2 and we will figure out where your error is.
     
  4. Oct 28, 2016 #3

    RBF

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    I used Δx/Δt to get vf with my initial time and displacement set as 0. 1000m/5s=200m/s. I calculated average acceleration using a=(vf-vi)/(tf-ti) initals values both at 0 (200m/s)/5s to get a=40m/s/s.
     
  5. Oct 28, 2016 #4

    phyzguy

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    Using Δx/Δt will give you the average velocity, not the final velocity. If the initial velocity is zero, the average velocity is 200 m/s, and the acceleration is constant, what is the final velocity?
     
  6. Oct 28, 2016 #5

    RBF

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    Not given, which made me rethink my approach. Should have used x=vi(t)+(1/2) at2 to solve for acceleration and then use that value for vf=vi+at.
     
  7. Oct 28, 2016 #6

    phyzguy

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    It doesn't need to be given. You should be able to figure it out given:
    (1) Initial velocity = 0
    (2) Average velocity = 200 m/s
    (3) Acceleration is constant

    That's how I would have done it.
     
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