Before attempting to solve problems that represent a physical phenomenum, it is important to understand the concept of a free body diagram. Often referred to as an FBD, a free body diagram is a simple representation used to show the relative magnitude and direction of all forces acting upon an object in a given situation.Free body diagrams are named as such because the diagram isolates the body from all other interacting bodies, and the diagram focuses only on the body of interest. The size of the vectors in a free-body diagram is reflective of the magnitude of the force. In certain cases, it is easier to write the magnitude of the force next to the vector being looked at, this way one does not have to worry about sizing it properly. The direction of the vector reveals the direction in which the force is acting. For simplicity, it is generally customary to represent an object by a box and to draw the force arrow from the center of the box outward in the direction that the force is acting. An example of a free-body diagram is shown at the right. (this section will have a pic wraped to the right)

It is important to be able to recognize what will be included and what will be excluded in a free body diagram. Below, there is a list of things to include and things to exclude when creating an FBD.

• Things to include:
  • A coordinate system
    • It is important to define a coordinate system before we begin sketching our FBD. This will make defining the vectors simpler when writing the equations of motion.
  • All external contacts, constraints, and body forces
    • These may include forces such as friction, gravity, normal force, drag, tension, or a human force due to pushing or pulling.

• Things to exclude:
  • Any external objects that are not part of the main object
  • Forces which the free body applies to other objects
    • If we are interested in a box resting on the ground, we would only be interested in the normal force that the ground is applying to the box and not the weight of the box. Notice that the two are equal and opposite, if we were to include them both, they would cancel each other out.
  • Internal forces
    • For example, if a bridge is being analyzed to find the reaction forces at the supports, the forces between the individual truss members would not be included.

Problem strategy is also an important concept to master when generating an FBD. Though it does not guarantee one will be able to solve the problem correctly, it certainly will point them in the right direction. Below we have four major rules that should be followed before jumping straight into a problem.

1. Read the problem

2. Carefully determine ALL the forces that are acting upon an object

3. Determine the direction and magnitude of each force

4. Go back and make sure everything was accounted for