Parallel force systems are those in which forces act in the same direction. The opposite of a parallel force system is a perpendicular force system, which is a system that has forces acting at right angles to each other.
An example of a parallel force system is a bag of oranges. While the oranges rest in the bottom of the bag, each is in a slightly different position. Accordingly, each has its own vector of force pulling it toward the earth’s center of gravity. However, the person carrying the bag does not feel all of these individual forces. Instead, the person carrying the bag feels the combination of all of the parallel forces as though they were pulling in one direction. The small differences between the force vectors even out, and the bag’s center of gravity appears to be drawn toward the earth.
Another example of a parallel force system occurs when a number of cars traverse over a bridge. Each of the cars is pulled toward the center of the earth by gravity. However, the bridge registers these forces as though they were all one force. This principle allows engineers to design bridges that allow all of the forces present to act as one.