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".
Answer
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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