What is Free Body Diagram : Definition with examples

free body diagram

Have you ever thought, how many forces are acting on a car when it is moving or what are the forces acting on you when you are running? A Free Body Diagram can help you to find and calculate those forces. Let’s learn more about the Free Body Diagram ( FBD) in this article.

What is a free body diagram ( FBD)? Free body diagram definition

Free Body Diagram is a simple schematic representation of various forces acting on an object when it is moving or stationary. A free body diagram is used to calculate static and dynamic forces acting on an object.

In other words, a free body diagram is the starting point to develop a mathematical model to find and calculate various forces acting on a body. The purpose of the free body diagram is to simplify the situation for easy analysis.

free body diagram
Figure :1

Why free body diagram is used?

  • To find out forces acting on a body
  • To find the direction of forces acting on a body
  • To simplify the model for easy analysis
  • To find relative forces acting on a body like frictional forces

Lets take an example of a car which is moving uphill. To calculate how much force required to push the car uphill, free body diagram can be used.

Let’s take another example of designing a leveler leg of a refrigerator. To design those legs we need to know how much force the refrigerator is putting on the ground. Based on that, we can design the leg. A free body diagram can be used to calculate that load.

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Free body diagram nomenclature

Before we learn about how to draw a free body diagram, we need to know about various nomenclature used to draw a free body diagram. Here are those.

Weight

Weight always acts perpendicular to the ground. If you know the mass of a body then the weight of a body would be W=mg, where ” g” is the gravitational force.

In the below image you have noticed that the weight of he block (mg) is acting perpendicular to the ground.

free body diagram

Normal Force

Normal force always acts perpendicular to the surface touching the object. Please make a note the object should touch the surface o have a normal force.

For example, if you are standing then your weight is acting downwards whereas a normal force is acting upwards or take the example of the block in the below figure. Here “N” denotes the normal force.

free body diagram

Friction Force

To understand frictional force let us look into the example in figure 1. In that example, the block is moving downwards. It is the friction force ( acting opposite to the motion) that is holding the block in place and not letting it go down. So to keep that block in place, friction force should be in the opposite of motion and more than the motion force.

There are two types of friction force. One is static friction and the other is kinetic friction. When a body is stationary, static friction is applied but when the body is moving, ten kinetic friction is applied.

Static friction is always higher than kinetic friction. In the image shown below ” F” is the friction force.

free body diagram

Tension

When a body is under two equal and opposite forces that try to elongate the body, then it is termed as tension on the body. For example, when you try yo stretch a rubber band, it is called tension force.

free body diagram

Compression

When a body is under two equal and opposite forces that try to compress the body then it is compressive force. For example, a gasket that is always under compression.

free body diagram

Net force

To understand net force let us again take the example in figure 1. In that figure, the body is moving downward and friction force is acting upwards. So if the friction force is greater than the motion force then the body won’t fall down but if the motion force is greater than the friction force then the body will fall. Between these two forces is called net force.

Net force is always applicable when there are two forces acting in the opposite direction.

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How to draw free body diagrams? Free body diagram worksheet

Let us take an example to draw the free body diagram. Let us assume that you are trying to push a block that is lying on the ground. To calculate the forces acting on that body we will use a free body diagram.

Step-1 : Identify all forces acting on the body

In this example following forces are acting on the body.

  • Weight of the block ( W) which is the product of mass and gravitational force
  • Reaction force or normal force ( R)
  • Push force ( P)
  • Friction force ( F)

Step-2 : Draw the schematic of the body

Draw the shape of the body and all other surfaces in the simplest form possible. Typically we use a rectangle when there are four sides of a body and for a circular shape, we use a circle. But frankly speaking, there is not such a role. It is the rule of the engineer to draw the diagram as simple as possible.

free body diagram

Step-3 : Show all forces with directions

Once the body is drawn, the next step is to draw force vectors to show how forces are acting. Use proper directional arrow for representation.

free body diagram

Step-4 : Calculate forces

The final step is to calculate the value of all those forces which are not known.

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Free Body Diagram examples

Let us take a simple example of an airplane and forces acting on it. Here the thrust and drag acting opposite to each other and the weight of aircraft and lift is acting opposite direction as well.

free body diagram

Conclusion

So I hope that you got a fair idea about the free body diagram. You might have understood that this is the basis of physics and as an engineer, we should have a good idea about how to draw a free body diagram.

If you have questions on free body diagram or stuck in drawing a free body diagram, do let me know in the comment section and I will be happy to assist.

Frequently Asked Questions ( FAQ)

What is free body diagram?

Free body diagram is a schematic representation of all forces acting on a stationary or moving body

What is the use of free body diagram?

Free body diagram is used to find and calculate all forces acting on a moving or stationary body.

How to draw free body diagram?

Step-1 : Identify all forces acting on the body
Step-2 : Draw the schematic of the body
Step-3 : Show all forces with directions
Step-4 : Calculate forces


Video Credit: Bozeman Science


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