Normal force: up or down? The normal force obviously always has direction perpendicular to the surface of contact, but I'm a bit confused about its sense: is it going 'up' or 'down'? I've seen articles on the web that describe the normal force as going either way.
My own reasoning would tell me that since force of friction is dependent on pressure — which usually translates to the normal force —, the normal force should go 'down'.
Why is it sometimes shown as going 'up'? Or is that just the normal reactive force?
 A: The normal force is a constraint force.  It is whatever it needs to be to keep an object in contact with a surface.  
Something like the top surface of the table I'm at now can only apply a normal force in the "up" direction.  If I push down on the laptop, the table will exert a normal force up to cancel my push and keep the laptop in the same place.  If I lift up on this laptop, the table will not exert a normal force down to cancel my lift; instead the laptop will accelerate up off the table.  So for this table and laptop, the normal force is up.  (Note, however, that this implies there is a normal force from the laptop on the table which points down.)
If I had something like a bead on a wire, the normal force would be the component of the force of the wire on the bead that points perpendicular to the tangent of the wire.  That could be in any direction, depending on the geometry of the wire.
A: It depends. If you have an object in a table, this table exert a normal force in order to keep the object in rest. In this case the normal force goes up. The object also exert a force to the table going down and related to pressure. And surely this is the origin of your confusion.
Now think about an object that you are putting against the roof whit your hand. In this case the roof exert to the object a normal force going down.
The normal force goes toward or away the surface in order to dont let the object pass through the constrain.  
