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?