Free Body Diagrams

Free Body Diagrams (F.B.D.)
A free body diagram is a simple sketch showing all the forces acting on an object when it is on its own (i.e. removed or isolated from its surrounding). Here are some guidelines for drawing Free Body Diagrams.

Forces are represented by vectors
A relative scale should be used to show the relative size of the forces whenever possible.
Only the forces acting on the object should be included not the forces that the object exerts on other objects or on the surroundings.
All forces (vectors) should be drawn with respect to the center of gravity (or center of mass of the object). This point is usually located at the geometric center of the object.

Example: Consider the apple hanging from the spring scale above. To draw a free body diagram for the forces acting on the apple we would have to consider the apple alone, removed from the spring scale.

Note the symbols used to represent the forces Fe is the elastic Force, Fg is the force of gravity. Note also the arrow head on top of the symbol indicating that the Force is a vector and finally note the direction of the forces; Fe is up while Fg is down.

Finally note that the spring scale and the apple are not moving. We say that they are in static equilibrium. When this happens the forces on an object are said to be balanced.

Forces acting on apple removed from spring scale