2. When two forces are combined, a parallelogram is made with the line segment representing the two forces as the adjacent side, and the diagonal of this parallelogram indicates the magnitude and direction of the resultant force, which is called the parallelogram rule.

3. 1726, johann bernoulli proposed in his letter to Hoarion that the parallelogram principle of force can be used in statics.

He used the principle of virtual work to analyze the problem of moment doing work in mechanical system, and pointed out that in any case of force balance, whether these forces are directly or indirectly used to support mutual balance.

5. daniel bernoulli put forward two questions about Hoarion in his article "Research on the Mechanical Principle and Proof of Decomposition and Synthesis of Force": When using synthesis and decomposition, force and speed should not be proportional.

6. Is the motion of an object independent under various forces?