Moment

Moment, in mechanics. If a force be applied to a body which either has one point in it fixed, or is attached by means of a rod to some other fixed point, that force will be unable to cause the body to move in any way, except to rotate round the fixed point. The amount of rotation which the force is capable of producing depends on the magnitude of the force, and on the perpendicular distance between the point and the direction of the force. The product of the force and the length of the perpendicular measures the rotation which the force can produce, and is termed the moment of the force about the point. If a line be drawn through the point perpendicular to the plane which contains the force and the point, that line will form an axis of rotation, and the above product is then called the moment of the force about this axis. It is convenient to regard moments as positive or negative, according as the rotation is clockwise i.e. in the direction in which the hands of a clock move - or contra-clockwise. The moment of a couple (q.v.) is the same for all points in space, and is equal to the product of one force and the perpendicular distance between the two. The moment of inertia of a body about an axis is the sum of the products obtained by multiplying the mass of each particle of the body by the square of its distance from the axis. This summation, as a rule, can only be done by the aid of the integral calculus.