What is Free Body Diagram and How to Draw FBD?

Free body diagram or FBD is a graphical representation of the relative magnitude and direction of all the forces acting on a body in given conditions. It is used to calculate forces acting on a body in moving and stationary conditions and solve static and dynamic mechanical engineering problems.

For example, FBD is used to calculate the force required to push a car on a hilly road or hold a book. Click this link to know in detail about the degree of freedom in mechanics.

This image shows the forces acting on a block on inclined plane.
FBD For Block Moving on Inclined Plane

As shown in the above image, the free body diagram of a rectangular block on an inclined plane shows various forces acting on the rectangular block. These forces can be used to understand the motion of the block in various conditions.

For example, when the value of frictional force is greater than the applied force (Friction Force > Applied Force), the block will remain in a stationary position. Whereas when the applied force becomes greater than frictional force (Applied Force > Friction Force), the block will start moving in the upward direction.

What is the purpose of Free Body Diagram in Physics and Engineering

FBD is a very important tool in physics and mechanical engineering to solve static and dynamic problems. Free Body Diagram is used for

  1. Visualizing forces acting on a moving or stationary body.
  2. Calculating unknown forces acting on a body.
Application Examples of FBD
  • FBD can be used to calculate the specification of a motor to lift a 50kg payload. It can be done by calculating the forces acting on the motor when it lifts 50 kg load in given conditions.
  • In robotics, FBD is used to calculate the actuation force required to perform a given task such as lift and drop a component. These forces are used to select required components.

How to Draw Free Body Diagrams ?

Let’s consider an example of a man trying to push a 25 kg wooden block on wooden flooring. We can draw FBD for wooden block in following three steps:

Man Pushing a Wooden Block

Step-1: Identify and list down all forces acting on a body.
This diagram shows List of Forces acting on a Block for free body diagram
List of Forces on Acting on Block

As shown above, when a man tries to push the wooden block. He works against the wooden block weight and frictional force in between wooden block and floor. Therefore following forces will act on the block.

  • Weight of the object (W).
  • Frictional Force (f) in the direction opposite to movement of block.
  • Ground Reactive Force (R)
  • Push force by Man (F) (for this example we will consider applied force is normal force)
Step-2: Create Rigid Body line diagram

Next step is creation of the object line diagram. Line diagram shows the basic shape of the body. In the case of a rectangle block, the line diagram will be a rectangle. Whereas the line diagram for the tennis ball will be a circle. 

Step-3:  Mark the forces acting on Rigid Body on Line Diagram
Free Body Diagram of a rectangle block when a man tries to push it.
Free Body Diagram For a Block

In the last step, FBD is drawn by drawing force vectors on rigid body line diagram. Force vectors are drawn using an arrow in the direction of acting force. As per Newton’s second law, in static condition all forces acting on a body are in equilibrium. Therefore we can calculate unknown forces by keeping opposite forces equal. For above example:

Push force = Frictional Force

Reaction Force = Weight of the block

In this way if frictional forces are known we can calculate the force required to move the block.

Free Body Diagram Examples

Here are the examples of FBD for products we use in our day to day life.

This image shows the Free body diagram of running player.
Running Person Free Body Diagram
This image shows the forces acting on a block on inclined plane.
FBD For Block Moving on Inclined Plane
Free body diagram of a book on table
FBD For Book on Table
This image shows free body diagram for free falling ball.
Free Falling Ball FBD

To sum up, FBD is a very good tool in physics and engineering to visualize and calculate the forces acting on a rigid body. We will keep updating more information on free body diagram. Please add your suggestions, comments or questions on FBD in the comment box.

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