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?