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Homework Help: Finding acceleration and tension on an incline

  1. Oct 5, 2013 #1
    1. The problem statement, all variables and given/known data
    Hi everyone, I hope you're enjoying your weekends!
    I've been trying this question for a while not having any luck, I'm hoping someone can tell me where I'm going wrong

    The question is:
    Two boxes are connected by a rope that passes through a pulley on the corner of a incline. Box A is 2.5 kg and box B is 5.5 kg. The coefficient of kinetic friction on the incline is 0.54. Box A is on the incline and box B hangs over. The angle of the incline is 25.4º. Find the acceleration and the tension.

    The answers given are 3.8 m/s2 [down] and 33 N [up]

    2. Relevant equations

    I got these equations from here:https://www.physicsforums.com/showthread.php?t=27346 and I understand why I would use them

    Ft = m1 *a + m1gcosθ + μm1gcosθ and
    Ft = m2(g-a)

    3. The attempt at a solution

    m1 = 2.5 kg, m2 = 5.5 kg, μ = 0.54, θ = 25.4°

    I know I can equate the two tension equations to get:

    m2(g-a) = m1 *a + m1gcosθ + μm1gcosθ

    But when I plug in the known values and solve:

    5.5(9.8-a) = 2.5*a + 2.5*9.8*cos(25.4) + 0.54*2.5*9.8cos(25.4)

    I get a value of a = 2.4

    Could someone tell me where I'm going wrong here?
    Thanks so much in advance!
  2. jcsd
  3. Oct 5, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    In that first equation, one of those cosines should be a sine.

    But please don't use someone else's derived equations. Derive them yourself, using Newton's 2nd law. Then you'll understand where they come from.
  4. Oct 5, 2013 #3
    Whoops, that's true!
    Thanks so much, I'll definitely be sure to in the future.
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