What is the meaning of moment? I'm little confused about the word as there are some terms like moment of momentum, moment of mass, moment of force, etc. I want to know what exactly is meant by the word "moment".
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$\begingroup$ Moment is often used as a synonym for torque. Moment of inertia is analogous to mass in the sense that it describes how much torque it takes to start (or stop) an object rotating. Moment of mass and moment of weight are not phrases I am familiar with. $\endgroup$– M. EnnsCommented Apr 24, 2016 at 18:31
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$\begingroup$ Related: physics.stackexchange.com/q/16389/2451 physics.stackexchange.com/q/44527/2451 $\endgroup$– Qmechanic ♦Commented Apr 24, 2016 at 19:05
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$\begingroup$ Also closely related: Why is a dipole moment called a dipole moment? $\endgroup$– Emilio PisantyCommented Apr 25, 2016 at 0:02
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$\begingroup$ Wikipedia on the origin of the term "moment". $\endgroup$– Emilio PisantyCommented Apr 25, 2016 at 0:27
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Like Wikipedia says: "Moment is a combination of a physical quantity and a distance." This 'physical quantity' could be various things. To take the examples you mention:
- Moment of momentum (commonly known as angular momentum) is expressed as $\vec{L}=\vec{r}\times m\vec{v}$, and is a measure for the rotational momentum of an object around some axis.
- Moment of mass (commonly known as moment of inertia) is expressed as $I = m r^2$, and is a measure the unwillingness of an object to accelerate in a rotational motion around some axis.
- Moment of force (commonly known as torque) is expressed as $\vec{\tau}=\vec{r}\times\vec{F}$, and is a measure of the tendency of a force to accelerate an object around some axis.
These quantities are basically analogs to the familiar concepts of momentum, mass and force in the case of rotational motion. Note that they are all defined relative to some axis: $r$ is the distance to the axis in each case.
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