Carl Richards, age 14, of Burlington, Vt., for his question:

CAN YOUR EXPLAIN TENSILE STRENGTH?

In engineering, tensile strength is the resistance of a body to tensile forces that tend pull it apart. The term is also used to define the maximum tensile stress, referred to as the force per unit cross sectional area (measured in kilograms per squre centimeter or pounds per square inch) that a body can resist before it ruptures.

In theory, the highest tensile strength would be exhibited by a single pure crystal in which the tensile strength would be the same as the force required to break the molecule apart. In practice, such strengths cannot be achieved because of the nonuniformities in the actual crystalline structure, which consists of many crystals, and also because of the impurities that are usually present.

The highest tensile strengths are achieved in practical engineering materials such as the carefully heat treated steel alloys, particularly if they are in the form of fine wires. In thesem tensile strengths of about 200,000 to 300,000 pounds per square inch are possible.

Cables for suspension bridges, for example, are made up of strands consisting of thousands of fine wires that have much higher tensile strength than thicker rods of the same overall cross section.