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Types of Forces
Forces that act on a body can be divided into two general categories—
Reaction
Reaction is the opposing force that a support offers whenever it is acted upon by external or inherent forces.
Free Body Diagram
Free body diagram is a diagram in which a rigid body is isolated from the system and all active forces applied to the body and reactive forces as a result of mechanical contact are represented.
Examples
Steps for Drawing Free Body Diagram 1. A sketch of the body is drawn assuming that all supports (surfaces of contact, supporting cables, etc.) have been removed. 2. All applied forces (including weight) and support reactions are drawn and labeled on the sketch. 3. Apply the weight of the body to its center of gravity (if it is uniform, then apply it to the centroid). If the sense of a reaction is unknown, it should be assumed 
A. EQUILBRIUM OF RIGID BODIES
A rigid body is said to be in equilibrium if the resultant of all external and reactive forces and moments acting on it is zero.
Lami’s Theorem
If three coplanar concurrent forces acting on a body keep it in equilibrium, then each force is proportional to the sine of the angle between the other two”
Note: Lami’s theorem is applicable only to 3 coplanar concurrent forces in equilibrium
Proof
By applying polygon law of forces, draw triangle OAB representing the system of forces shown with external angles as indicated
Applying Sine Law
General Equations of Equilibrium
1. The algebraic sum of all forces in a force system is zero.
2. The algebraic sum of all moments in a force system is zero.
Equations of Equilibrium For Coplanar Systems
Solving Equilibrium Problems 1. Draw proper Free Body Diagram 
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