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

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

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**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

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**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.

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**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.

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**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.

Do you have any suggestions? Please write in
comment box and also drop your email id in the given mail box which is given at
right hand side of page for further and continuous update from

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

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**Reference: **

Engineering Mechanics, By Prof K. Ramesh

Image courtesy: Google

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