Free Body Diagrams
A free body diagram is a tool used to solve engineering mechanics problems. As the name suggests, the purpose of the diagram is to "free" the body from all other objects and surfaces around it so that it can be studied in isolation. We will also draw in any forces or moments acting on the body, including those forces and moments exerted by the surrounding bodies and surfaces that we removed.
The diagram below shows a ladder supporting a person and the free body diagram of that ladder. As you can see, the ladder is separated from all other objects and all forces acting on the ladder are drawn in with key dimensions and angles shown.
Constructing the Free Body Diagram
The first step in solving most mechanics problems will be to construct a free body diagram. This simplified diagram will allow us to more easily write out the equilibrium equations for statics or strengths of materials problems, or the equations of motion for dynamics problems.
To construct the diagram we will use the following process.
- First draw the body being analyzed, separated from all other surrounding bodies and surfaces.
- Second, draw in all forces and moments acting directly on the body.
Do not include any forces acting on the other bodies in the system. Some
common types of forces
seen in mechanics problems are:
- Gravitational Forces: Unless otherwise noted, the mass of an object will result in a gravitational weight force applied to that body. This weight is usually given in pounds in the English system, and is modeled as 9.81 (g) times the mass of the body in kilograms for the metric system (resulting in a weight in Newtons). This force will always point down towards the center of the earth and act on the center of mass of the body.
- Normal Forces (or Reaction Forces): Every object in direct contact with
the body will exert a normal force on that body which prevents
the two objects from occupying the same space at the same time.
Note that only objects in direct contact can exert normal forces
on the body.
- An object in contact with another object or surface will experience a normal force that is perpendicular (hence normal) to the surfaces in contact.
- Joints or connections between bodies can also cause reaction forces or moments, and we will have one force or moment for each type of motion or rotation the connection prevents.
- Friction Forces: Objects in direct contact with the
body can also exert friction forces on the body, which will resist
the two bodies sliding against one another. These forces will
always be perpendicular to the surfaces in contact. Friction is
the subject of an entire chapter in this book, but for simple
scenarios we usually assume rough or smooth surfaces.
- For smooth surfaces we assume that there is no friction force.
- For rough surfaces we assume that the bodies will not slide relative to one another no matter what. In this case the friction force is always just large enough to prevent this sliding.
- Tension in Cables: Cables, wires or ropes attached to the body will exert a tension force on the body in the direction of the cable.
- The above forces are the most common, but other forces such as pressure from fluids, spring forces and magnetic forces may exist and may act on the body.
- Once the forces are identified and added to the free body diagram, the last step is to label any key dimensions and angles on the diagram.