Free body diagrams are sketches used to analyse the forces and moments that act on structures and machines and drawing them is an important skill.
The free body diagram identifies the mechanical system being examined, and it represents all of the known and unknown forces present.
Follow three main steps when drawing a free body diagram:
Select an object that will be analyzed by using the equilibrium equations. Imagine that a dotted line is drawn around the object and note how the line would cut through and expose various forces. Everything within the dotted line is isolated from the surroundings and should appear on the diagram.
Next, the coordinate system is drawn to indicate the positive sign conventions for forces and moments. It is meaningless to report an answer of -25 N or +250 lb without having defined the directions associated with the positive and negative signs.
In the final step, all forces and moments are drawn and labeled.
These forces might represent weight or contact between the free body and other objects that were removed when the body was isolated. When a force is known, its direction and magnitude should be written on the diagram.
At this step in the analysis, forces are included even if their magnitudes and directions are not known.
After applying the equilibrium equations and consistently using a sign convention, the correct direction will be determined from your calculations.
If you find the quantity to be positive, then you know that the correct direction was chosen at the outset.
On the other hand, if the numerical value turns out to be negative, the result simply means that the force acts counter to the assumed direction.