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Force.
When we push or pull a body, we exert force on it. Force is a vector, i.e., it is described in terms of size and direction. We find that:.
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The overall force (the "weighted- force) applied to a body, is equal to the sum of forces acting on it. .
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The word sum requires particular attention: since force is a vector, we must consider the direction of the force examined. We must perform what is called vector addition, which takes direction into consideration: If forces are acting in the same direction, we do normal addition (think of two horses pulling a cart!). But, if the forces are acting in opposite directions, we subtract one from the other. In any given situation, between these two poles (same and opposite directions), we must weigh the forces (this is the overall force).
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This can be done by referring to the parallelogram law. Any two forces can be added by drawing a parallelogram, in which the two adjacent sides represent the forces (the length of each is determined by its numerical value). The (weighted) sum of the two forces is the diagonal of the parallelogram. .
Take the following example. Two people are pushing an object using different forces: one toward the northeast, the other, northward. .
Mass and Weight.
The force of gravity is of particular importance. This force, which causes bodies to fall toward earth, endows them with an extremely important attribute: weight. As the force of gravity increases, so does the body's weight. A body that weighs 100 newtons [2] on earth would weigh only 17 newtons on the moon. The symbol for weight is W. Its direction is the same as that of the force of gravity (which is the same as that of gravitational acceleration, g). Weight changes according to the force of gravity applied to it. However, the body itself does not change. Thus, the task of pushing a truck along a smooth surface on the moon is no less difficult than it is on earth. Therefore, we need a measure that will express this constant attribute, and does not depend on the force of gravity.
