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Force

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Force

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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.

The Editors of Encyclopaedia Britannica

This article was most recently revised and updated by

Erik Gregersen

, Senior Editor.

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