Write important guidance for solving problems in mechanics.

(N/A) We can solve different problems of dynamics using Newton's three laws of motion.
Sometimes,a problem involves more than one body,and these bodies exert forces on each other.
Moreover,every body experiences a gravitational force. To solve such problems,we must define a 'system' whose motion is to be discussed,while the remaining parts and external agencies are considered the 'environment'.
To solve the problem,follow these steps:
$(1)$ Draw a schematic diagram showing the assembly of different objects,their connections,and supports.
$(2)$ Select the object as the system whose motion you want to analyze. If the system consists of multiple objects,ensure that the acceleration vector of all these objects is the same.
$(3)$ List all forces acting on the system by the environment. Do not include internal forces acting within the system.
$(4)$ Represent the system as a point and draw all external forces acting on it as vectors originating from that point. This is called a Free Body Diagram $(FBD)$. This does not mean the system is free from forces,but only that the forces acting $ON$ the system are shown.
$(5)$ If necessary,select another system and repeat the steps. Use Newton's third law: for example,if the force exerted by $B$ on $A$ is $\vec{F}$ in the $FBD$ of $A$,then the force exerted by $A$ on $B$ in the $FBD$ of $B$ is $-\vec{F}$.