1，F =F 1+F2。

2.F =F 1-F2 [Calculation of resultant force of two opposite forces on the same line]. If there is an included angle a between F 1 and F2, it is obtained by cosine theorem that F =√(F 1? +F2？ -2F 1F2cosa).

pay attention to

If two forces are not linear, the diagonal direction is the direction of the resultant force.

If the two forces are in the same direction, the resultant force is equal to the sum of the two forces, and the direction remains the same.

If two forces are in 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.

If both forces are balanced forces, the resultant force is zero. ∑F=0 .