Concept of Force and its Features in Engineering Mechanics

A force can simply be defined as

"Its an agent which produces or tends to produce, destroys or tends to destroy motion"

To enhance the understanding, some helpful examples are as; when we see a horse applying force to pull a cart and to set it in motion or even a couple of magnets  attracting or repelling each other. Apart from those examples one can think of many examples by observing our day to day life.  

The feature and effects of Force can be listed as :  

• A force is a vector quantity, that means it has a magnitude and a direction as well.
• A force represents the action of one body on another.
• Force can be exerted by actual contact or at a distance (Gravitational, Magnetic force etc.) 
• It may change the motion of a body (i.e if a body is at rest, the force may set in motion. And if the body is already in motion it may accelerate it.)
• It may retard the motion of a body.
• It may retard the forces, already acting on a body, thus bringing it to rest or in equilibrium.
• It may give rise to internal stresses in the body, on which it acts.

Characterization of Force : 

A force is characterized in order to determine its affects. There are mainly Four such characteristics of a force, and they are :

• Point of application of the force
• Magnitude of the force 
• Direction of the force 
• Nature of the Force(Push or pull). This is denoted by placing an arrow head on the line of action of the Force.  

LEARNING POINTS :

1. In statics we consider a force which tends produce a motion. Sometimes the applied force may not be sufficient to move a body. Like if we try to lift a body of a weight;  beyond our capacity, and we fail to do so. In that case we have exerted a force undoubtedly, but the weight appears to be motionless. This kind of act teaches us that a force necessarily may not produce a motion in the body; but it may, simply tend to do so.
2. In Newtonian mechanics, Space, Time and Mass are absolute concepts and they are independent of each other (Not applicable in relativistic mechanics). On the other hand the concept of force is independent of the other three.

Unit and Dimension of Force : 

The SI Unit of Force is NEWTON represented as N. That also can be defined as 

"The amount of force when applied to a 1 KG weight body, then the body will attain an acceleration of 1 m/s^2".

The  CGS Unit of Force is DYNE. "A unit of force that, acting on a mass of 1 Gram, increases its velocity by 1 Centimeter per Second every second along the direction that it acts". 

To learn the dimension of a Force, its unit comes handy. Though unlike the unit, the dimension of force (or any other property) is fixed, but its easy to derive the dimension from the definition of the unit.

The unit of force can be unfolded as, 

Hence, the Dimension of force is derived from its SI unit as above.   

Continuing in the journey of learning statics; one need to understand two important ideas. They are the ''Particle" and the "Rigid body".

  

Concept of Particle : 

In statics, its important to underscore the significance of particle and 

"It may be defined as a body of infinetly small volume and is considered to be a concentrated point". 

Here the term infinetly small volume indicates that the physical presence of such a particle is not possible, rather the concept is ideal and valid for mathematical analysis only.  

Concept of Rigid Body :

"A Rigid Body may be defined as a body which can retain its shape and size, even if subjected to some external forces"

Like the particle concept, there is no body which is perfectly rigid in actual practice. But for the sake of simplicity, we take all the bodies as rigid bodies in mathematical analysis and modelling works. 

 A Rigid Body can be imagined as a combination of large number of particles occupying fixed positions with respect to each other. The study of mechanics of particles  is obviously prerequisite to that of Rigid Bodies.

The Physical Independence Principle of Force :

The principle of physical independence gives the hint to the idea of a "Resultant", as it states that

"If a number of force is simultaneously acting on a particle, then the resultant of these forces will have the same effect as produced by all the forces"

It clearly indicates and teaches us that; the combined effects of multiple forces can be seen in one separate force as a resultant. And its affect can be seen in practice along with its magnitude and direction.     

The study of elementary mechanics rests on six fundamental principles based on experimental evidence. And they are listed as :

1. The Parallelogram Law for the addition of forces.
2. The Principle of Transmissiblity.
3. Newtons First Law
4. Newtons Second Law
5. Newtons Third Law
6. The Newtons Law of Gravitation.       

We will continue enjoy learning the principles in detail and in more elaborate manner in the coming blog. Please do share and comment in the comment box below about the article and suggest your ideas to make the presentations better.  

References :

1. "Vector Mechanics for Engineers, Statics" by Ferdinand P Beer, E. Russell Johnson, David F. Mazurek, Phillip J Cornwell, Elliot R Eisenberg & Sanjeev Sanghi.
2. "Materials Science and Engineering and Introduction" by William D Callister Jr, David G Rethwisch
3. "A Textbook of Engineering Mechanics" by R.S Khurmi