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Homework Help: Beginner physics question

  1. Sep 18, 2006 #1
    1Dim. Physics question

    I'm in my 3rd weeks of physics, so I'm dealing with Kinematic Equations.

    My question is: A train accelerates from an initial velocity of 25.0 km/hr to a final vel. of 65.0km/h in 8.5 seconds..Find its acceleration and the distance the train travels during this time.

    for acceleration, I did A = Final Vel - Initial Vel / time and when put in m/s , it comes out to 1.31 meters per second squared...i checked the answered in the back of the book..it checks out..ok, i got that part...

    now... for distance, I used X = 1/2(final vel + initial Vel)t no matter how many times I did it, my answer doesnt match up with the books correct answer of 106m

    i also thought i might have used the wrong equation so i tried using:

    X=Final Vel(time) + 1/2at squared

    still doesnt come out right

    what am i doing wrong? it's really frustrating, ...I need to get this and do real well because its part of my pre-med requirement, so any help would be appreciated..thanks
    Last edited: Sep 18, 2006
  2. jcsd
  3. Sep 18, 2006 #2
    X=Final Vel(time) + 1/2at squared
    you didn't copy this equation correctly
  4. Sep 18, 2006 #3
    sorry, I meant

    x= initial vel*time +1/2at squared
  5. Sep 19, 2006 #4
    ok, and you used this equation and it did not work?
    can you write out the exact equation (with the numbers you used) to solve for the second part?
  6. Sep 19, 2006 #5
    x= initial velocity(time) + 1/2at squared

    x = 25(8.5) + 1/2(1.31)(8.5 squared)
    x= 212.5 + .655(8.5squared)
    x= 212.5 + 72.91
    x= 285.41

    yet, x is supposed to be 106 in the book
  7. Sep 19, 2006 #6
    You should be using 25000m/3600s for your initial velocity, not 25. That'll get you 106. :)
  8. Sep 19, 2006 #7


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    Homework Helper

    x = initial position + initial vel*time + 1/2at squared. Irrelevant in this problem, but somewhere else it just may not be. :smile:
  9. Sep 19, 2006 #8
    ahhh thank you everyone, really appreciate it
  10. Sep 19, 2006 #9
    damn...i used Oksanav's advice and heres where im stuck

    x= 1/2 (25,000m/3,600s + 65,000m/3,600s)(8.5s)

    x= 1/2 (90,000m/7,200s) (8.5s)

    now heres where im gettin stuck...im multiplying (90,000m/7,200s) by (8.5s) and getting 106.25 and then taking half of that because of the '1/2' at the beginning of the equation....the answer in the book is 106 though..so what did i do wrong

    x= 1/2 (106.25) = 53.125??
  11. Sep 19, 2006 #10


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    Gold Member

    Hello toddler,

    you are not applying the correct equation in your last post.

    You've already found out in post #5 that:


    with a being



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