1, when two forces act on the same straight line with the same direction, the resultant force is equal to the sum of the two forces with the same direction; Mathematical expression: f =F 1+F2.

2. When two forces act on the same straight line with opposite directions, the resultant force is equal to the difference between the two forces, and the direction is the same as that of the larger force; Mathematical expression: f =F 1-F2.

3. When two forces act on different straight lines, follow the parallelogram rule or triangle rule.