Mechanics are often called upon to determine the size of rod or beam required to support a certain weight or force. Not all pieces of material have the same strength. The strength of any piece of material depends upon the nature of the material (cohesion of the particles composing it) and upon the position, shape, and bulk of the piece.. Therefore, it is absolutely necessary to know something about the properties and laws governing the strength of materials used in industry. When a force acts on beams, structures, or bodies of any kind, it may be considered as weight, and may be measured in pounds.