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Why is the acceleration positive?

  1. Sep 24, 2011 #1
    1. The problem statement, all variables and given/known data
    Two blocks attached by a string slide down a 17 incline. The lower block has a mass of 0.2 kg and a coefficient of kinetic friction of 0.2. The upper block has a mass of 0.9 kg and a coefficient of kinetic friction of 0.9. Find the magnitude of the acceleration of the blocks. The acceleration of gravity is 9.81 m/s2 .

    2. Relevant equations
    uk = kinetic coefficient
    Fx1 = m1a = m1gsin 17 - T - uk1 * m1gcos 17
    Fx2 = m2a = m2gsin 17 + T - uk2 * m2gcos17

    3. The attempt at a solution
    a(m1+m2) = (m1 + m2)gsin 17 - uk1 * m1gcos 17 - uk2 * m2gcos 17
    a = -4.3765924 m/s^2

    However, the answer was a = 4.3765924 m/s^2.
    It seems like the equation was:
    Fx1 = m1a = T + uk1 * m1gcos 17 - m1gsin 17
    Fx2 = m2a = uk2 * m2gcos 17 - m2gsin 17 - T
    The equation was flipped, but I do not understand why. Since blocks are sliding down, shouldn't friction have the negative value because it is going against acceleration??
  2. jcsd
  3. Sep 24, 2011 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Regardless of whether you've set up your equations correctly or not, (which I haven't checked), this word in red above is important. Your answer is going to be positive, because the question asks you to state the magnitude of the acceleration vector only, not its direction.
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