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Find speed and acceleration of truck

  1. Sep 21, 2007 #1
    1. The problem statement, all variables and given/known data
    a truck covers 40m in 8.50s while slowing down to a final speed of 2.80m/s


    2. Relevant equations

    find original speed
    find it's acceleration

    3. The attempt at a solution

    for original speed i think it is 40m/8.50s= 4.7m/s
    now for acceleration i am not sure which kinematic equation to use. i think it's Vf=Vi+at am i on the right track or no
     
  2. jcsd
  3. Sep 21, 2007 #2

    learningphysics

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    That's not right... for original speed.

    d = vt only applies for constant speed situations...

    you need a different equation to find the original speed.
     
  4. Sep 22, 2007 #3
    so is the original speed then the same as the initial speed
     
  5. Sep 22, 2007 #4
    yes, what equation do you know that relates velocity, initial velocity, and time?
     
  6. Sep 22, 2007 #5
    well for acceleration you would use a= (Vf-Vi)/t
    for initial speed would you use a=2(x-V1t)/t^2
     
  7. Sep 22, 2007 #6

    learningphysics

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    Yes, but the acceleration formula here requires initial speed... the initial speed formula requires acceleration... so neither of these formulas let you get started...

    you can calculate original speed immediately. which formula lets you get it?
     
  8. Sep 24, 2007 #7
    can u set these two equations equal to each other, cancel acceleration and solve for Vi or am I making it more complicated
     
  9. Sep 24, 2007 #8

    learningphysics

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    d = (v1+v2)/2 * t

    You can use this to get the original speed.
     
  10. Oct 1, 2007 #9
    i get it ..thank u
     
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