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Homework Help: Acceleration 1d kinematics

  1. Oct 11, 2006 #1
    Starting from rest, a speedboat reaches a speed of 3.4 m/s in 1.8 s. What is the boat's speed after an additional 2.6 s has elapsed, assuming the boat's acceleration remains the same?

    I have tried finding the velocity for the first 1.8 seconds

    [tex]v = v_0 + a t[/tex]

    3.4 = 0 + a(1.8)

    3.4 / 1.8

    a = 1.889

    Then tried finding the final velocity assuming it was the same, but using 3.4 as the initial velocity..

    v = 3.4 + (1.8)(3.6)

    Didn't work.

    I also tried v = (1.8)(4.4) adding the total time together, and using v0 as 0.
     
  2. jcsd
  3. Oct 11, 2006 #2

    radou

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    Homework Helper

    It's v = 3.4 + 1.8*2.6
     
  4. Oct 11, 2006 #3
    Yeah that's a typo on my part, I meant to type 2.6

    The velocity still comes out to 8.08 and that is incorrect.
     
  5. Oct 11, 2006 #4
    Well, I get 8.31 s, and if that is not the answer, then Houston, we have a problem....
     
  6. Oct 11, 2006 #5

    How do you get 8.31?
     
  7. Oct 11, 2006 #6
    I always use 3 significant figures, 3.4/1.8 = a =1.889, not 1.8, then round to 2 at the end.

    So, (1.8 + 2.6)s is the elapsed time which equals 4.40s. Then, you multiply that by 1.889m/s/s to get 8.31m/s.
     
    Last edited: Oct 11, 2006
  8. Oct 11, 2006 #7
    I guess that was my problem, wasnt doing enough figures.
     
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