Moment (physics)

Classical mechanics

$\mathbf{F} = \frac{\mathrm{d}}{\mathrm{d}t}(m \mathbf{v})$
Newton's Second Law

History of classical mechanics · Timeline of classical mechanics

Branches
Statics · Dynamics / Kinetics · Kinematics · Applied mechanics · Celestial mechanics · Continuum mechanics · Statistical mechanics

Formulations
Newtonian mechanics (Vectorial mechanics) Analytical mechanics: Lagrangian mechanics Hamiltonian mechanics

Fundamental concepts
Space · Time · Velocity · Speed · Mass · Acceleration · Gravity · Force · Impulse · Torque / Moment / Couple · Momentum · Angular momentum · Inertia · Moment of inertia · Reference frame · Energy · Kinetic energy · Potential energy · Mechanical work · Virtual work · D'Alembert's principle

Core topics

Rigid body · Rigid body dynamics · Euler's equations (rigid body dynamics) · Motion · Newton's laws of motion · Newton's law of universal gravitation · Equations of motion · Inertial frame of reference · Non-inertial reference frame · Rotating reference frame · Fictitious force · Linear motion · Mechanics of planar particle motion · Displacement (vector) · Relative velocity · Friction · Simple harmonic motion · Harmonic oscillator · Vibration · Damping · Damping ratio · Rotational motion · Circular motion · Uniform circular motion · Non-uniform circular motion · Centripetal force · Centrifugal force · Centrifugal force (rotating reference frame) · Reactive centrifugal force · Coriolis force · Pendulum · Rotational speed · Angular acceleration · Angular velocity · Angular frequency · Angular displacement

Scientists
Isaac Newton · Jeremiah Horrocks · Leonhard Euler · Jean le Rond d'Alembert · Alexis Clairaut · Joseph Louis Lagrange · Pierre-Simon Laplace · William Rowan Hamilton · Siméon-Denis Poisson

"Principle of Moments" redirects here. For the Robert Plant album, see The Principle of Moments. For a more abstract concept of moments that evolved from this concept of physics, see Moment (mathematics).

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.
Moment of momentum or angular momentum ( $L = I \omega\!$ ) is the rotational analog 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 ( $\mathbf{p}=q \mathbf{d}$ )