A free body diagram is a pictorial representation often used by physicists and engineers to analyze the forces acting on a free body. Free body is the generic term used by physicists and engineers to describe some thing&mdashbe it a Bowling ball, a Spacecraft, Pendulum It shows all contact and non-contact forces acting on the body. In Physics, a contact force is a force between two objects (or an object and a surface that are in contact with each other A non-contact force is any force applied to an object by another body that is not in direct contact with it In Physics, a force is whatever can cause an object with Mass to Accelerate. Drawing such a diagram can aid in solving for the unknown forces or the equations of motion of the body. Creating a free body diagram can make it easier to understand the forces, and moments, in relation to one another and suggest the proper concepts to apply in order to find the solution to a problem. In Physics, the moment of force (often just moment, though there are other quantities of that name such as Moment of inertia) is a Pseudovector The diagrams are also used as a conceptual device to help identify the internal forces, (for example shear forces and bending moments in beams), which are developed within structures. 
The free body diagram starts with a sketch or just an outline of the free body. All external contacts and contraints are left out.
All external contacts, constraints, and body forces are replaced by vectors, representing the different forces acting on the object. The vectors show the direction and magnitude of the various forces. The magnitude of a mathematical object is its size a property by which it can be larger or smaller than other objects of the same kind in technical terms an Ordering To the extent possible or practical, the vectors should indicate the point of application of the force they represent.
Only the forces acting on the object are included. These may include forces such as friction, gravity, the normal force, drag, or plain old contact force due to pushing. Friction is the Force resisting the relative motion of two Surfaces in contact or a surface in contact with a fluid (e Gravitation is a natural Phenomenon by which objects with Mass attract one another In Physics, the normal force F_n\ (or in some books N) is the component perpendicular to the surface of contact of the Contact force In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a When in a non-inertial reference frame, fictitious forces may be appropriate. A non-inertial reference frame is a frame that is not an Inertial reference frame. A fictitious force, also called a pseudo force, d'Alembert force or inertial force, is an apparent Force that acts on all masses in a non-inertial Each vector should point in the direction of the force it represents, and be labeled with the magnitude of that force.
Forces which the free body applies to other objects are not included. For example, if a ball rests on a table, the ball applies a force to the table, and the table applies an equal and opposite force to the ball. The FBD of the ball only includes the force that the table causes on the ball.
A coordinate system is usually included, according to convenience. In Mathematics and its applications a coordinate system is a system for assigning an n - Tuple of Numbers or scalars to each point This may make defining the vectors simpler when writing the equations of motion. The x direction might be chosen to point down the ramp in an inclined plane problem, for example. This article deals with the physical structure For related terms see Canal inclined plane, Cable railway, Funicular, or Fixed-wing In that case the friction force only has an x component, and the normal force only has a y component. The force of gravity will still have components in both the x and y direction: mgsin(theta) in the x and mgcos(theta) in the y, where theta is the angle between the ramp and the horizontal.
A simple free body diagram, shown above, of a block on a ramp illustrates this.