Free Body Diagrams
A crate like those commonly used in the shipping industry is resting on two smooth blocks and has a weight W, as seen in the figure below (left).
The Free Body Diagram of the crate has been drawn to the right of the original problem diagram. In the FBD, the blocks have been replaced with two reaction forces, R1 and R2 and the weight of the crate, W, has been indicated.In solving mechanics problems, often it is necessary to fully understand what external forces are acting on an object of interest. Free Body Diagrams (FBD's) are simple sketches of the object that show all forces that act on the object including reaction forces which replace all of the object's supports.
The correct procedure for drawing an FBD consists of the following essential steps:
- Decide which object is "of interest" to the problem at hand.
- Make a sketch of the object, isolating it from its constraints.
- Draw and label all forces acting on the body i.e. support reaction forces, weight, applied forces, etc.
- Choose and indicate a set of coordinate axes and place any relevant dimensions on your diagram.
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