F 1 and F2 are two components, and θ is the included angle between F 1 and F2. It is also correct to change cosθ to cos(π-θ) (-cosθ=cos(π-θ), so the two formulas are equivalent). The basic method of multi-force synthesis is still parallelogram rule. Specifically, first choose two components and try to calculate their resultant force, and then try to work with the third component with the obtained results until the sum of all components is tried.
When using the law
Some commonly used mathematical knowledge: trigonometric function method, trigonometric rule, sine theorem, similar triangles method. This method is often used to analyze the dynamic changes of vectors. Especially when the sum vector is unchanged and the direction of one vector is unchanged, it is more suitable for analyzing the size and direction change of another vector! In fact, this method is a variant of the parallelogram rule, but it is more widely used than the parallelogram method.