Application of Newton's Laws
FREE-BODY DIAGRAMS
Rev. 0
Page 5
CP-04
FREE-BODY DIAGRAMS
In studying the effect of forces on a body it is necessary to isolate the body and
determine all forces acting upon it. This method of using a free-body diagram is
essential in understanding basic and complex force problems.
EO 1.2
STATE the purpose of a free-body
diagram.
EO 1.3
Given all necessary information, CONSTRUCT a
free-body diagram.
In solving a problem involving forces it is essential that Newton's laws are carefully fixed in
mind and that these principles are applied literally and exactly. In applying these principles
it is essential that the body be isolated from all other bodies so that a complete and accurate
account of all forces which act on this body may be considered. The diagram of such an
isolated body with the representation of all external forces acting on it is called a Free-Body
Diagram. It has long been established that the free-body-diagram method is the key to the
understanding of engineering problems. This is because the isolation of a body is the tool that
clearly separates cause and effect and focuses our attention to the literal application of a
principle.
Example:
Consider the book resting on the table in Figure 1. Although the book is
stationary, two forces are acting on the book to keep it stationary. One is the
weight (W) of the book exerting a force down on the table. The other is the
force exerted up by the table to hold the book in place. This force is known
as the normal force (N) and is equal to the weight of the book. A normal
force is defined as any perpendicular force with which any two surfaces are
pressed against each other. The free-body diagram for this situation,
illustrated on the right side in Figure 1, isolates the book and presents the
forces acting on the object.
