# Engineering mechanics | Free Body Diagram

## Engineering mechanics :

* “Engineering mechanics* describes the behavior of a body, in either a beginning state of rest or of motion of rigid body, subjected to the action of forces”.

Of course, engineering mechanics is an integral component of the education of engineers whose disciplines are related to the mechanical behavior of bodies or fluids. Such behavior is of interest to aeronautical, civil, chemical, electrical, mechanical, metallurgical, mining and textile engineers. A sound training in engineering mechanics continues to be one of the most important aspects of engineering education due to the interdisciplinary character of many engineering applications (e.g. spaceship, robotic and manufacturing). It is appropriate to conclude that the subject of engineering mechanics is the core of all engineering analysis.

### Classification of *engineering mechanics* :

Engineering mechanics may be divided into following two main branches :

1. Statics 2. Dynamics

1. Static : It is that branch of engineering mechanics, which deals with the forces and their effects, while acting upon the bodies at rest.

2. Dynamics : It is that branch of * engineering mechanics*, which deals with forces and their effects, while acting upon body in motion.

Dynamics may be further subdivided into the following sub‐branches :

(i) Kinetics : It is that branch of dynamics which deals with the bodies in motion due to the application of forces.

(ii) Kinematics : It is that branch of dynamics which deals with the bodies in motion. Without any reference to the forces which are responsible for the motion.

The study of kinematics assimilates terms such as displacement, velocity, acceleration, retardation etc. which are important to an engineer in the design of moving part of a machine.

* Newton’s first law (the law of inertia) : *The law states that; “A body will remain at rest or in uniform motion in a straight line unless it is compelled to change this state by forces impressed upon it.”

The first law depicts that if there is no external effect, an object must be still or moving at a constant velocity, which leads to a concept of the net force.

* Newton’s second law (the law of motion) :* This law states that; “A body acted upon by an external unbalanced force will accelerate in proportion to the magnitude of this force in the direction in which this force acts.”

The second law quantifies the force in terms of acceleration and mass of the object.

F = ma

* Newton’s third law (the law of action and reaction) : *This law states that, “For every action (or force) there is an equal and opposite reaction (or force)”.

The third law illustrates the existence of the counter force which is related to normal forces, tension, etc.

## Free Body Diagram (FBD):

If a body consist of more than one element and support, each then element and support can be isolated from the system, such diagram is called * free body diagram*.

One of the most useful aids for solving a statics problem is the free body diagram (FBD). A free body diagram is a graphic, dematerialized, symbolic representation of the body (structure, element or segment of an element) in which all connecting ʺpiecesʺ have been removed. A FBD is a convenient method to model the structure, structural element, or segment that is under scrutiny. It is a way to conceptualize the structure, and its composite elements, so that an analysis may be initialized.

To draw FBD of a body we remove all the supports (like wall, floor, hinge or any other body) and replace them by the reactions which these supports exert on the body.