# Moment (physics): Wikis

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# Encyclopedia

In physics, the term "moment" can refer to many different concepts:

• Moment of force (often just moment) is the tendency of a force to twist or rotate an object; see the article torque for details. This is an important, basic concept in engineering and physics. (Note: In mechanical and civil engineering, "moment" and "torque" have different meanings, while in physics they are synonyms. See the discussion in the "torque" article, or the article couple (mechanics).)
• Moment arm is a quantity used when calculating moments of force. See the article torque.
• The Principle of moments is a theorem concerning moments or force. See the article torque.
• A pure moment is a special type of moment of force. See the article couple (mechanics).
• Moment of a vector is a generalization of the moment of force. The moment M of a vector B about the point A is
$\mathbf{M_A} = \mathbf{r_{AB}} \times \mathbf{B}$
where
$\mathbf{r_{AB}}$ is the vector from point A to the position where quantity B is applied.
× represents the cross product of the vectors.
Thus M can be referred to as "the moment M with respect to the axis that goes through the point A", or simply "the moment M around A". If A is the origin, or, informally, if the axis involved is clear from context, one often omits A and says simply moment.
When B is the force, the moment of force is the torque as defined above.
• Moment of inertia ($I = \sum m r^2$) is analogous to mass in discussions of rotational motion.
• Angular momentum ($L = I \omega\!$) is the rotational analog of momentum. (Historically, angular momentum was sometimes referred to as "moment of momentum".)
• Magnetic moment ($\mathbf{\mu}=I\mathbf{A}$) is a dipole moment measuring the strength and direction of a magnetic source.
• Electric dipole moment is a dipole moment measuring the charge difference and direction between two or more charges. For example, the electric dipole moment between a change of -q and q separated by a distance of d is ($\mathbf{p}=q \mathbf{d}$)

# Simple English

In physics, the moment of force (often just moment) means the magnitude of force that influences the rotation of an object.

In this concept the moment arm, the distance from the axis of rotation, plays an important role. The lever, pulley, gear, and most other simple machines create mechanical advantage by changing the moment arm. The SI unit for moment is the newton meter (Nm).

## History of moment

The principle of moments is derived from Archimedes' discovery of the operating principle of the lever. In the lever one applies a force, in his day most often human muscle, to an arm, a beam of some sort. Archimedes noted that the amount of force applied to the object, the moment of force, is defined as M = rF, where F is the applied force, and r is the distance from the applied force to object.