We were discussing various important topics based on the basics of centrifugal compressor and hydraulic turbines in our previous posts.

Today we will be interested here to start here a very important subject i.e. engineering mechanics with the help of this post. We will secure here the various principles and concepts of engineering mechanics such as basic concepts of force i.e. Concept of force system in engineering mechanics, understanding of free body diagram, equilibrium of forces and rigid bodies, basics of supports, basics aspects of friction, basics of trusses etc.

So, let us start here our first important concept of engineering mechanics i.e. force system in engineering mechanics. We will find out here the following concepts with the help of this post i.e. force system in engineering mechanics.
1. Concept of a force and effect of a force
2. Newton’s Third law of motion
3. Unit of force

### Concept of force system in engineering mechanics

A force is basically the action of one body on another body which changes or tends to change the motion of the body or state of the body.

The effect of a force on a body or object will be the combination of translation motion i.e. linear motion and rotational motion.

Suppose if someone will push us at the centre of our chest and if we do not resist against the pushing force, what will be the motion of our body? Think it. Our body will move or translate backward linearly.

Suppose if someone will push us at the one edge of our chest and if we do not resist against the pushing force, what will be the motion of our body? Think it again. Our body will move or translate backward linearly as well as tend to rotate.

Therefore, in general any force acting on an object will produce the combination of translation motion i.e. linear motion and rotational motion.

A force is a vector quantity and hence it will be represented by a vector. Force will be characterized by following points as mentioned below.
1. Point of applications
2. Magnitude of force
3. Direction of force

As we are well aware with the Newton’s third law of motion, we can understand here that force will always exist in the pairs.

### Newton’s third law of motion

Newton’s third law of motion tells us that for every action there will be an equal and opposite reaction.

Let us see here two objects 1 and 2 as displayed here in following figure.

Let us assume that force exerted by object 1 on object 2 is F12 and force exerted by object 2 on object 1 is F21

According to Newton’s third law of motion, force exerted by the object 1 on object 2 will be equal and opposite in the direction with the force exerted by the object 2 on object 1.

Mathematically, we can write here as
F12 = - F21

Therefore, force will always exist in the pairs. We must note it that action and reaction forces will always act on different objects.

### Force interactions

Once we draw the free body diagram of a system where various forces are involved, force interaction will be very important to consider during the drawing of free body diagram.

Force interactions could be between systems in direct contact or those which are physically separated.

Let us consider the following figure, where an object is placed over the table. Hence, there is direct contact between the table and object.

We can also think the case of aeroplane going in the sky. Aeroplane flying in the sky will have also an interaction between the earth even there is no physical contact between the aeroplane and earth.

### Unit of force

Unit of force is Newton and we represent it as N.

Recall the equation or formula for force derived from the Newton’s second law of motion.

F = m x a
Where, m is the mass of object in Kg and a is the acceleration of the object in m/s2 provided due to the application of force F.

Let us assume that mass i.e. m is 1 kg and acceleration i.e. a produced due to the application of force F is 1 m/s2. Think the above situation and we will have the information about the force F and it will be 1 N.

Therefore, we can define a Newton as a force that develops an acceleration of 1 m/s2 to a mass of 1 kg.

Therefore, we have introduced here the basics of engineering mechanics where we have seen various basic concepts in engineering mechanics such as concept of force system, unit of force, effect of force and Newton’s third law of motion.

Further we will see other concepts of engineering mechanics such as principle of transmissibility of forces, classification of force systems in engineering mechanics etc.

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Further we will find out, in our next post, principle of transmissibility of forces and its limitations.

### Reference:

Engineering Mechanics, By Prof  K. Ramesh