Moment (physics)

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

• Moment of force (often just moment) is a synonym for torque, an important basic concept in physics, civil engineering, and mechanical engineering. In the context of mechanical engineering, the terms are not necessarily interchangeable, but one or the other may be preferred in a specific context. For example, "torque" is usually used to describe a rotational force down a shaft, for example a turning screw-driver, whereas "moment" is more often used to describe a bending force on a beam.
  • Moment arm is a quantity used when calculating torque. See the article torque.
  • The Principle of moments is a theorem concerning torques. See the article torque.
• Moment of a vector is a generalization of the moment of force. The moment M of a vector B about the point A is

where

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 () is analogous to mass in discussions of rotational motion.
• Angular momentum (L = Iω) is the rotational analog of momentum. (Historically, angular momentum was sometimes referred to as "moment of momentum".)
• Magnetic moment () is a dipole moment measuring the strength and direction of a magnetic source.