# Direction of force of friction?

I know that friction opposes the direction of the velocity, but the problem is when I don't know what the direction of the velocity is. When I'm drawing a FBD for complex systems with ramps and pullies and stuff (with angular forces considered), I don't really know if the mass is accelerating/moving down or up and hence I don't know if the friction is to the left or to the right.

How do I figure it out? Is there any tips you have for me for figuring that out? Thanks in advance.

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In most cases it doesn't matter. Thee signs that you get will tel you. – jinawee Jan 22 '14 at 18:24

The way to think about it is, imagine the friction as a "reaction force". It reacts to the motion. If the mass hanging from the pulley was overwhelmingly heavier than the mass on the ramp, it'll obviously pull the ramp mass up and thus friction would be trying to oppose this (and vice versa). So first solve the problem, assuming there's no friction, and this will give you the direction it will move. Then, account for friction, and you may now find that it still moves in that direction, or the friction is big enough that it doesn't move at all (also, don't make the common mistake students often do: sometimes they find that the frictional force is bigger than the accelerating force and then add them up and find that friction is pushing the mass in a strange direction! But the frictional force you get from $F_{fric} = \mu F_N$ is a maximum, the most friction can do. Thus if the term above is bigger than the accelerating force, only a force of magnitude equal to the accelerating force acts in the opposite direction to it).