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Homework Help: Circular motion

  1. Sep 1, 2007 #1
    1. The problem statement, all variables and given/known data

    car is on a horizontal circular track of 80m radius and car starts from rest to 100km/h in 10 seconds. what's the magnitude a of ttotal acceleration of the car 8 seconds after start?

    2. Relevant equations

    normal acceleration = v^2/r

    3. The attempt at a solution

    convert v to m/s and r = 80. then i found normal acc.

    i just have no clue on how to do this...class notes and book reading dont help me at all.
    what are things i need to consider?
    thank you
  2. jcsd
  3. Sep 1, 2007 #2
    there are two accelerations:
    tangential, and normal.

    dividing this problem into two parts might help you.
  4. Sep 1, 2007 #3
    tangential is what's messing me up...
    books says it's equal to v with dot above it (derivative)
    or also equal to r*theta(with dot above as well)

    i dont get what they actually mean or how to get it...
  5. Sep 1, 2007 #4
    well, I am assuming here that tangential a is uniform.
    Just ignore them, I guess you really don't have an equation to differentiate.

    just use that simple for: a = vf-vi/t
    10 seconds, and assume that it's same for 8 seconds.

    I am pretty sure that this would be correct
  6. Sep 1, 2007 #5
    Last edited: Sep 1, 2007
  7. Sep 1, 2007 #6


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    Homework Helper

    Yes, you have found the acceleration. But the velocity that you use in the a = v^2/r equation must be the velocity of the car at t = 8 s, NOT at t =10 s, or Vf. You must use the acceleration you just found to find the velocity of the car at t = 8 s.

    Also, the total acceleration is the vector sum of the tangential and centripetal accelerations.
  8. Sep 1, 2007 #7
    well can i use v = vo+at ? considering that a is constant and vo is zero(?)
    how can you use vector to do this problem...i dont know how to set up the vector...
    no coords :(

    [edit] well i can sum up and get final vectors i think...only problem is how to get normal acc. and tang acc. in vector form...
    Last edited: Sep 1, 2007
  9. Sep 1, 2007 #8
    alright, here's what i have...

    v = vo + at (given that vo = 0, v = 100km/h, t = 10)

    i get 2.78m/s^2 as acceleration.

    v = vo+2.78t (given that vo=0, t = 8)

    i get v = 22.2m/s^2

    an = v^2/r (given that v = 22.2 and r = 80)

    i get an = 6.17m/s^2 and it should be NEGATIVE.

    for the vector sum up part...since they are perpendicular to each other, i just find magnitude of two acceleration which is...

    sqrt(((-6.17)^2)+(2.78^2)) = 6.77m/s^2

    answer is right but just confirming if this is how i should do it.

    thank you so much!
    Last edited: Sep 1, 2007
  10. Sep 2, 2007 #9


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    Homework Helper

    Yes, that looks right to me.
  11. Sep 2, 2007 #10
    ok thank you very much :)
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