Free Body Diagrams

How would you keep track of the different forces acting on this quadcopter?

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What is a free body diagram?

The very first step in solving structural engineering problems is to draw what is known as a free body diagram.  If you have taken physics, you may have done this already!  This is a simple process and is very helpful in modeling physical systems.  Let’s start with an example, picture a car.  

Draw a sketch of the car you’re picturing (do not worry if art is not your thing, the goal here is to capture the physics, this is not an art contest).

Now draw arrows to represent the vectors (magnitude and direction) of the forces that act on the car.  For example, the car has weight, so you should have a vector pointed downward.  But the car doesn’t fall through the ground, so there is a normal force from the earth upwards on the car that balances out weight.  Is your car moving?  If so, then there is probably a force between the tires and the road.  Similarly, there’s friction between the tires and the road acting in the opposite direction along with air drag.  Is your car accelerating?  Newton’s second law says the sum of the forces on a body equal’s the body’s mass times its acceleration.  So, if the body is accelerating, then the force between the tires and the road should be larger than the drag and frictional forces.  Need a little practice with this?  Check out the Free Body Diagram Interactive at: https://www.physicsclassroom.com/Physics-Interactives/Newtons-Laws/Free-Body-Diagrams/Free-Body-Diagram-Interactive or view our video below on drawing a free body diagram of a tuna blimp.

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Next Steps

Now that you can draw free body diagrams, you are ready to start understanding how structures come together. Begin with Trusses.

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Last updated: November 23, 2022.