Free Body Diagram

 

Free Body Diagram(F.B.D.) :- In a system when several bodies are connected by strings, springs, surfaces of contact, then all the forces acting on the body are sketched by isolating the body from its surroundings. This diagram is called free body diagram.

In order to draw F.B.D., the following points are kept in mind :-

1. When two blocks (say A and B) are connected by a string, then the tension in the string is taken same at every point on it and it is considered away from the body under consideration.

 

In the above figure tension T for block A is towards block B and for block B it is towards block A.

2. The reaction force (R) also called Normal Reaction (N) always acts normal to the surface on which the body is kept.

3. When a block is connected by a spring and the spring is stretched by a distance x, then force on block F = kx ( opposite to stretching of spring, where k is the spring constant of the spring).

Steps for Free Body Diagram(F.B.D.)

 

Step 1 :- Identify the object or system whose F.B.D. is required and isolate it from other objects clearly specifying its boundary.
Step 2 :- First draw all the non-contact external force in the diagram, like weight, electrostatic force etc.
Step 3 :- Draw contact forces acting at the boundary of the object or system like normal reaction, friction, tension and external applied force, etc.

Note :-

1. While drawing F.B.D. always take into account the forces which are acting on the body. Never take into account the forces which the body is exerting on others.
2. In F.B.D, internal forces are not drawn, only external forces are drawn.

 

Example 1. A block of mass ‘m’ is kept on the ground as shown in figure :

(i) Draw F.B.D. of the block.
(ii) Are forces acting on block action–reaction pair ?
(iii) If answer is no, draw action–reaction pair.

Solution : (i) F.B.D. of block :-

(ii) “N” and “mg” are not action-reaction pair. Since action-reaction pair act on different bodies and they are of same nature.

(iii) Action-reaction pair of mg :

And action-reaction pair of ‘N’ acts on the surface as shown :