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There is a box on a ramp, there is Wx that makes it fall, Tension that pulls it up the ramp (a rope) and Friction.

The thing is, how can I determine the direction of the friction? For me, friction was always opposed to the movement, but in this case it is in the direction of movement (seeing the solution, the box is climbing the ramp with constant velocity).

Acceleration is zero in this case, so yeah... How do I determine friction direction when acceleration is zero?

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    $\begingroup$ Friction cannot provide a force in the direction of movement. It always opposes movement. I suspect that there is some aspect of either the problem or the solution that you have misunderstood. $\endgroup$
    – BowlOfRed
    Commented Feb 12, 2015 at 6:42
  • $\begingroup$ You are right, I should have read the problem again, I'm ashamed... thanks for your answers and I promise to be more careful in the future. $\endgroup$
    – Vidarr
    Commented Feb 13, 2015 at 3:03

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The direction of kinetic friction is in the opposite direction of the velocity of the box and independent from acceleration thus whether the $\mathbf a$ is positive, negative or zero does not affect the direction of the friction vector

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Static friction will always oppose the resulting force before friction is considered. If the downward force is larger than the upward one, friction will be upward. Else, if the upward force is larger, friction will point downward.

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