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Homework Help: Finding intial speed

  1. Nov 27, 2013 #1
    1. The problem statement, all variables and given/known data

    A frictionless plane is 11.4 m long and inclined
    at 39.4◦. A sled starts at the bottom with an
    initial speed of 4.46 m/s up the incline. When
    it reaches the point at which it momentarily
    stops, a second sled is released from the top
    of this incline with an initial speed vi. Both
    sleds reach the bottom of the incline at the
    same moment.The acceleration of gravity is 9.8 m/s

    Find the initial speed of the second sled.
    Answer in units of m/s

    2. Relevant equations

    x = (1/2) * a * t² + v0 * t

    3. The attempt at a solution

    t = 4.46/6.22
    t = .717
    So basically all I did was plug and chug...

    11.4 = .5 * 9.8 * sin39.4 * .717^2 + v0 * .717

    12.998 = v0 * .717
    v0 = 18.129

    Some reason I'm not getting the correct answer. Anyone want to tell me what i'm doing wrong? Thanks.
  2. jcsd
  3. Nov 28, 2013 #2


    Staff: Mentor

    First you should draw a picture of the incline the sleds and the force of gravity affecting the sleds.

    From the initial problem statement you know that the time to reach the bottom is the same for both sleds.

    And you know that the same gravity force is acting on them.

    The other thin you know is that one sled starts falling from some point on the incline which isn't at the top but the second sled is definitely at the top of the incline.

    Lastly, you know the second sled has an initial velocity but you don't know what it is.

    So draw a picture and think about these relationships and the formulas you have to work with then post your results again.
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