THE WHAT, WHY AND HOW OF A FREE BODY DIAGRAM (FBD)
Free Body Diagrams are one of the most important things for you to know how to draw and use.
What ? - It is a drawing that shows all external forces acting on the particle.
Why ? - It helps you write the equations of equilibrium used to solve for the unknowns (usually forces or angles).
How ?
1. Imagine the particle to be isolated or cut free from its surroundings.
2. Show all the forces that act on the particle.
Active forces: They want to move the particle. Reactive forces: They tend to resist the motion.
3. Identify each force and show all known magnitudes and directions. Show all unknown magnitudes and / or directions as variables .
Fundamental Principles
•The parallelogram law for the addition of forces: Two forces acting on a particle can be replaced by a single force, called resultant, obtained by drawing the diagonal of the parallelogram which has sides equal to the given forces
•The principle of transmissibility: A force acting at a point of a rigid body can be replaced by a force of the the same magnitude and same direction, but acting on at a different point on the line of action
