Write important guidance for solving problems in mechanics.

We can solve different problems of dynamics using Newton's three laws of motion, which we have learned.

Sometimes in a given problem more than one body is involved such bodies exert force on each other.

Moreover every body also experiences gravitational force. In solving such problems, out of the assembly of bodies or systems. We have to take that part as a system of which the motion is to be discussed and the remaining parts of the assembly and other agencies which affect our system are taken as environment.

To solve the problem we should proceed according to the following steps :

$(1)$ Draw a schematic diagram showing the assembly of different object, other objects connected with them and those which support these objects.

$(2)$ Select the object as system of which we want to discuss the motion. If the system under consideration is made up of more than one object, then take care that the acceleration vector of all these objects should be the same.

$(3)$ Make a list of the forces acting on the system by the remaining parts of the assembly and by other agencies. In this list the forces acting inside the system are not to be included.

$(4)$ Showing a system as a point all the forces acting on it are depicted as vectors from that point. This figure is called the Free Body Diagram (FBD). This does not mean that the system under consideration is free from forces actually, only the forces on the system are shown in the figure.

$(5)$ If necessery, you will take another system and apply steps as above. Use Newton's third law. For example in FBD of $A$, the force exerted by $B$ on $A$ is shown as $\vec{F}$, then in FBD of $B$ the force exerted on B by A can be shown as $-\vec{F}$.