System :

Two or more than two objects which interact with each other form a system.

3.1       Classification of forces on the basis of boundary of system :

(A)        Internal Forces:

Forces acting each with in a system among its constituents.

(B)        External Forces:

Forces exerted on the constituents of a system by the outside surroundings are called as external forces.

(C)        Real Force:

Force which acts on an object due to other object is called as real force. An isolated object (far away from all objects) does not experience any real force.

Free Body Diagram: A free body diagram consists of a diagrammatic representations of a single body or a subsystem of bodies isolated from its surroundings showing all the forces acting on it.

Steps for making  F.B.D.

Step 1:Identify the object or system and isolate it from other objects clearly  specify its boundary.

Step 2: First draw non-contact external force in the diagram. Generally it is weight.

Step 3:Draw contact forces which acts at the boundary of the object or system. Contact forces are normal, friction, tension and  applied force.

Remember N F T G E

In F.B.D, internal forces are not drawn, only external are drawn.

Question:  Two sphere A and B are placed between two vertical walls as shown in figure. Draw the free body diagrams of  both the spheres.

Answer

FBD of shere A

FBD of sphere B