Sunday, 24 November 2019

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 systemprinciple of transmissibility of forces and its limitations, classification of force systembody force and surface force, moment of a force, Varignon’s theorem and concept of a couple in engineering mechanics.

Now, we will be interested here to understand how to draw free body diagrams in engineering mechanics with the help of this post.

How to draw free body diagrams in engineering mechanics

This topic i.e. how to draw free body diagrams is very important to understand for solving any kind of mechanics problems involving the different types of forces.

We will understand here the basics of free body diagram, definition of free body diagram and we will also find out the method to draw the free body diagram for a system.

So, let us first see here the definition of free body diagram

Free body diagram

A free body diagram is basically a sketch or drawing of the selected system consisting of a body, a portion of a body or a collection of interconnected bodies completely isolated or free from all other bodies, showing the interaction of all other bodies by forces on the one being considered.

Let us consider the following figure that indicates two discs placed inside a box.

Depending on the requirement of given problem, we can consider one disc as system, second disc as system or both discs together as system or we can also consider the box as system as displayed here in following figure.

Important points need to consider during drawing a free-body diagram

Free body diagram is basically a diagram or sketch of the system under consideration.

In drawing the free body diagram, we need to show the system completely isolated form all other bodies including foundations, supports etc.

During drawing the free body diagram, we will show the interaction on the system by each body with the help of forces.

We will show all the forces i.e. known forces and unknown forces in free body diagram. Unknown forces will be determined by following the principle of equilibrium.

We will have to show the magnitude and direction of external forces in free body diagram.

We will show the unknown reaction forces in free body diagram.

We will assume the direction for unknown reaction forces and if our final answer comes with positive sign then it indicates that our assumption for the direction for force is correct. If our final answer comes with negative sign then it indicates that our assumption for the direction for force is incorrect and it will be just opposite to what we have assumed.

We need to indicate the actual direction for unknown frictional forces.

Let us see here one example to understand how to draw the free body diagram for a given problem

Following figure indicates the physical conditions of the problem. We will draw here the free body diagram for this problem.

A free body diagram, as displayed here in following figure, is basically a sketch that indicates only the forces on the selected particle or system.

Therefore, we have seen here the basics of free body diagram and we have also seen here how to draw the free body diagram for a given problem in engineering mechanics with the help of this post.

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Further we will find out, in our next post, resultant of the forces acting on a body in engineering mechanics.

Reference:

Engineering Mechanics, By Prof K. Ramesh
Image courtesy: Google