This extends from Newton's first law of motion. A common physics lab is to hang an object by two or more strings and to measure the forces that are exerted at angles upon the object to support its weight.
The most common application involves the analysis of the forces acting upon a sign that is at rest.
For example, consider the picture at the right that hangs on a wall.
The leftward pull of cable A must balance the rightward pull of cable B and the sum of the upward pull of cable A and cable B must balance the weight of the sign.
Suppose the tension in both of the cables is measured to be 50 N and that the angle that each cable makes with the horizontal is known to be 30 degrees. This question can be answered by conducting a force analysis using trigonometric functions.
We would have to conclude that this low margin of experimental error reflects an experiment with excellent results.
We could say it's "close enough for government work." The above analysis of the forces acting upon an object in equilibrium is commonly used to analyze situations involving objects at static equilibrium.
The object is a on a string upon which three forces were acting. If the object is at equilibrium, then the net force acting upon the object should be 0 Newton.
Thus, if all the forces are added together as vectors, then the resultant force (the vector sum) should be 0 Newton.