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Kinematics Acceleration

  1. Jul 27, 2016 #1
    1. The problem statement, all variables and given/known data
    A sled starts from rest and slides down an inclined plane with constant acceleration. In three seconds, it has travelled 9m. How long does it take to reach a velocity of 24m / s?

    2. Relevant equations
    d = vi * t + ½ * a * t^2, vi = 0

    vf = vi + a * t, vi = 0

    3. The attempt at a solution
    d = vi * t + ½ * a * t^2, vi = 0
    9 = ½ * a * 3^2
    a = 18 ÷ 9 = 2 m/s^2
    To determine the time, we need to determine the sled’s acceleration.

    vf = vi + a * t, vi = 0
    24 = 2 * t
    t = 12 seconds.

    I solved this problem with help of someone else . The thing I don't understand is why can't we use 9,81 m/s2 for acceleration . Because the sled is sliding down . Doesn't it has to do with gravity ?
     
  2. jcsd
  3. Jul 27, 2016 #2
    You can't use 9.81 because the ramp is not completely vertical. Only a portion of the acceleration due to gravity is accelerating the sled
     
  4. Jul 27, 2016 #3
    But this can be a 2D motion right ? with horizontal and vertical components ?
     
  5. Jul 27, 2016 #4
    yes, it can.
     
  6. Jul 27, 2016 #5
    To explore it using horizontal and vertical components you would need the angle of the plane though
     
  7. Jul 27, 2016 #6
    Oh thats right . The angle is indeed not given . Thank you very much . I know it was a very dumb question .
     
  8. Jul 27, 2016 #7
    not a dumb question at all
     
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