Free Body Diagram (FBD)

Sketching Free Body Diagrams (FBDs) is probably one of the most important skill you can acquire as an engineer.   Believe it or not, solving most engineering problems begin with a FBD.

To use the equation of equilibrium we must be aware of all the forces acting on the object.  If you're dealing with one or two forces then maybe it's easy to just visualize your problem but more often than not we are looking at problems with many many forces (well not that many) so it is best to put pen to paper.

These are the simple steps of producing an Free Body Diagram

1. The best way to start a FBD is to sketch the object we are analyzing in such a way that it is isolated from its surrounding.  
2. Once you have an outline you can start inserting all the known and unknown forces acting on the object.
3. Lastly, all the forces identified should be labeled.

• Known forces are labeled with its magnitude and direction
• As for unknown forces, its magnitudes are represented by alphabets while it's direction can be directed randomly (upon solving the problem, it's true direction will become known)

That's it for this post, stay tuned for my next post when I show you some Free Body Diagram sketching examples.

Condition for Equilibrium

An object is in equilibrium if it satisfies Newtons' first law:

"An object at rest or moving in a straight line with constant velocity will remain in this state unless it is subjected to an unbalanced force"

Quite simply put, an object is in equilibrium if the sum of forces acting on it equals zero:

This equation is called the equation of equilibrium.

Engineering Mechanics (Statics) - Equilibrium of a Particle

Before we discuss "equilibrium" lets go back a little and see what we understand about the whole subject of Mechanics.

Mechanics is a subject that is concerned with what happens to the motion of an object when forces are acting on it.  Mechanics can be divided into three branches¹:

1. Rigid-body mechanics (under the assumption that forces acting on the object cause no deformation upon the object)
2. Deformable-body mechanics
3. Fluid mechanics

At this point in time we are concerned with rigid-body mechanics which can be further divided into two areas:

1. Statics
2. Dynamics

When we talk of equilibrium we are in the Statics area of rigid-body mechanics.

An object is said to be in equilibrium when it is static or moving at constant velocity (not accelerating)

Dynamics on the other hand deals with object that are accelerating.