yield stress

mechanics

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elasticity

In stress
Yield stress, marking the transition from elastic to plastic behaviour, is the minimum stress at which a solid will undergo permanent deformation or plastic flow without a significant increase in the load or external force. The Earth shows an elastic response to the stresses caused…
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mechanical properties testing

In metallurgy: Testing mechanical properties
…most common mechanical properties are yield stress, elongation, hardness, and toughness. The first two are measured in a tensile test, where a sample is loaded until it begins to undergo plastic strain (i.e., strain that is not recovered when the sample is unloaded). This stress is called the yield stress.…
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physical metallurgy

In metallurgy: Mechanical properties
…however, a point called the yield stress will be reached and exceeded. Strain will now increase faster than stress, and, when the sample is unloaded, a residual plastic strain (or elongation) will remain. The elastic strain at the yield stress is typically 0.1 to 1 percent, whereas, with the sample…
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force

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Two forces applied simultaneously to the same point have the same effect as a single equivalent force. The resultant force can be found by constructing a parallelogram with the initial force vectors forming two adjacent sides. The diagonal of the parallelogram gives the resultant force vector.

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force

physics

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Article History

Key People:: Isaac Newton; Christiaan Huygens; Sir Charles Vernon Boys

Related Topics:: electromagnetism; gravity; fundamental force; wear; friction

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force, in mechanics, any action that tends to maintain or alter the motion of a body or to distort it. The concept of force is commonly explained in terms of Isaac Newton’s three laws of motion set forth in his Principia Mathematica (1687). According to Newton’s first principle, a body that is at rest or moving at a uniform rate in a straight line will remain in that state until some force is applied to it. The second law says that when an external force acts on a body, it produces an acceleration (change in velocity) of the body in the direction of the force. The magnitude of the acceleration is directly proportional to the magnitude of the external force and inversely proportional to the quantity of matter in the body. Newton’s third law states that when one body exerts a force on another body, the second body exerts an equal force on the first body. This principle of action and reaction explains why a force tends to deform a body (i.e., change its shape) whether or not it causes the body to move. The deformation of a body can usually be neglected when investigating its motion.

Because force has both magnitude and direction, it is a vector quantity. The representation of forces by vectors implies that they are concentrated either at a single point or along a single line. This is, however, physically impossible. On a loaded component of a structure, for example, the applied force produces an internal force, or stress, that is distributed over the cross section of the component. The force of gravity is invariably distributed throughout the volume of a body. Nonetheless, when the equilibrium of a body is the primary consideration, it is generally valid as well as convenient to assume that the forces are concentrated at a single point. In the case of gravitational force, the total weight of a body may be assumed to be concentrated at its centre of gravity (see gravity, centre of).

Physicists use the newton, a unit of the International System (SI), for measuring force. A newton is the force needed to accelerate a body weighing one kilogram by one metre per second per second. The formula F = ma is employed to calculate the number of newtons required to increase or decrease the velocity of a given body. In countries still using the English system of measurement, engineers commonly measure force in pounds. One pound of force imparts to a one-pound object an acceleration of 32.17 feet per second squared.

Italian-born physicist Dr. Enrico Fermi draws a diagram at a blackboard with mathematical equations. circa 1950.

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The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Erik Gregersen.