When two forces P and Q act along the same line, their resultant force is simply the sum or difference of the two forces, depending on their direction:

* If the forces act in the same direction: The resultant force is the sum of the two forces: R = P + Q.

* If the forces act in opposite directions: The resultant force is the difference between the two forces: R = |P - Q| (where | | denotes absolute value).

Example:

If P = 10 N and Q = 5 N, then:

* Same direction: R = 10 N + 5 N = 15 N

* Opposite direction: R = |10 N - 5 N| = 5 N

Important Note: The resultant force will always act along the same line as the original forces.