Learn To Engineer

Learn To Engineer

Free Body Diagrams

A Free Body Diagram is a diagram of an object that is being analysed with all the forces applied to it drawn. Simply put: draw the object, draw the forces on it (gravity etc) then replace objects around it with forces eg something on top of it becomes a point force.

The Free Body Diagram (FBD) is a very useful tool in engineering and drawing one is often the first step in solving many problems. Here you will be shown how to draw Free Body Diagrams of different objects/situations.

Step 1: Understanding the problem

You have to understand the problem before you can begin to draw it. You may need to make a few sketches of the of the situation before you can fully understand what is happening. Once you understand this, you can start to draw some free body diagrams.
Some problems required more than one free body diagram, because each free body diagram can only be used for one object in the question, you may need to analyse several objects in a single situation.

Step 2: Seperate the important object from the rest

You know what object you want to draw a free body diagram of, so the diagram should only include this object and none of the other ones. You can simplify this object as much as you want to, for example a car can often be represented by a rectangle or a beam represented as a straight line.

Step 3: Draw already known forces on the object

Generally you have things such as gravity and friction acting on an object, the source of these forces is not generally shown (well for gravity at least) in the original picture of the question. You need to put these forces onto the diagram first. They are drawn simply as vectors.

Step 4: Replace other objects with force vectors

Generally problems will require more than one object, and these objects all apply forces to each other such as the normal force applied to an object from the 'ground'. when adding these forces into the diagram it is important to take into account all the forces. Generally you will cross off one external object from your sketch then add one or more forces to the FBD to ensure that all external objects have been examined.

Here is an example question: Drawing a FBD