Force is defined as a vector quantity that tends to produce an acceleration of a body in the direction of its application. Changing the body's velocity causes the body to accelerate. Therefore, force can be mathematically defined as given by Newton's second law of motion (Equation 4-1).

F = ma (4-1)

where:

Force is characterized by its point of application, its magnitude, and its direction. A force that is actually distributed over a small area of the body upon which it acts may be considered a concentrated force if the dimensions of the area involved are small compared with other pertinent dimensions.

Two or more forces may act upon an object without affecting its state of motion. For example, a book resting upon a table has a downward force acting on it caused by gravity and an upward force exerted on it from the table top. These two forces cancel and the net force of the book is zero. This fact can be verified by observing that no change in the state of motion has occurred.