We have started a new topic in our previous post i.e. engineering mechanics. We have seen there the basics of engineering mechanics such as concept of force system, unit of force, effect of force and Newton’s third law of motion

Now we will be interested to understand here the principle of transmissibility of forces and classification of force systems in engineering mechanics. We will first understand the principle of transmissibility of forces with the help of this post.

### Principle of transmissibility of forces

Principle of transmissibility states that the state of rest or of motion of a rigid body will be unaltered if a force acting on the body will be replaced by another force of the same magnitude and direction but acting anywhere on the body along the same line of action of the applied forces.

In other words, we can also write here the principle of transmissibility as the external effects of the force will be independent of the point of application of the force along the line of action of the force.

### Let us understand here with the help of following example.

Let us consider here two rigid blocks A and B as displayed here in following figure. Let us assume that these two rigid blocks are joined with each other with the help of a rigid rod as shown in the figure.

Let us think that a force F is applied over the object B and hence system i.e. both rigid blocks A and B and the rigid rod will move towards right direction. In other words, we are applying here the pulling force and system is moving towards right direction under the action of this pulling force F.

Now, we have replaced the force with same magnitude and direction i.e. F and the force is acting now on the object A but in the same line of action.

What will be the external effect of this force? Let us see here in the following figure.

System i.e. rigid blocks A and B with rigid rod will move in right direction. In other words, we are applying here the pushing force and system is moving towards right direction under the action of this pushing force F.

Therefore, we have noted here one very important observation i.e. pulling force or pushing force will produce the same effect.

### Let us see the following figure…

There is one rigid object and two forces of same magnitude are acting on the rigid object as displayed here in figure. What will be the condition or state of the object?

Object will be stationary, while it will be under tension load.

If we move the force along the line of action, object will now be under compression as displayed here in following figure. Again, the body will be stationary.
Therefore, the external effects will be same in both the cases.

### Limitations of principle of transmissibility of forces

The principle of transmissibility of forces will be only applicable for rigid bodies. It will not be applicable for deformable bodies.

Now, let us consider the following figure where we are considering one deformable body i.e. the distance between the particles of the body will vary under the load.

When we will apply the tensioning load, there will be increase in the length of the body or object as we have considered it as deformable body.

When we will apply the compressive load, there will be reduction in the length of the body or object as we have considered it as deformable body.

Therefore, we have studied here the principle of transmissibility of forces and we have also discussed here the limitations of principle of transmissibility of forces with the help of this post.

Further, we will see the classification of force systems in engineering mechanics with the help of our next post.

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Further we will find out, in our next post, classification of force systems in engineering mechanics

### Reference:

Engineering Mechanics, By Prof K. Ramesh

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