Simple Forces: Add/Subtract for Net Force?

[p4cifico]

I am confused as to when I should be able to tell "by observing" whether the forces of normal, kinetic friction, and tension need to be added or subtracted from the net force for both sideways and vertical directions. Ropes pulling at degrees, pulleys moving up and down, cars traveling uphill, how do I know when to subtract or add these forces from $$\Sigma$$ F (x or y) ?

 

[TVP45]

Almost always, you can tell which way gravity points. And, ropes/strings/wires always pull in the axial direction. Friction always opposes motion (or a tendency toward motion). Normal means perpendicular to a surface.

Then, if you use standard angle notation, you always add all forces; if a force magnitude comes out negative, add 180 degrees to the "guessed" angle. BTW, if by chance you don't know standard angle, ignore this posting.

 

[Doc Al]

...how do I know when to subtract or add these forces from $$\Sigma$$ F (x or y) ?

I will assume that you know how to determine (by inspection) the actual direction of a force and how to find its components. (TVP45 gave some advice in that regard.) As far as whether you add or subtract a given force when finding the net force, that depends on your sign convention. For example, consider vertical forces. If you take the usual convention where up is positive and down is negative, then any downward force will be negative when adding vertical forces. For horizontal forces, to the right is often positive and to the left is negative.