# Confused about the terminology moment

## Main Question or Discussion Point

Hello,

I am confused about the terminology moment, specifically that there is a 1st moment and 2nd moment. What does moment even mean, and why does it have numbers associated with them?

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Drakkith
Staff Emeritus
What context is it being used in?

torques

jhae2.718
Gold Member
Are you referring to the 1st Moment of Area and 2nd Moment of Area?

If so, these are relations based on the distributions of points in an area, e.g.:
1st Moment of Area:
$S_x = \int_A y {\rm d}A$
(This is often used to find the centroid of an object.)

2nd Moment of Area:
$I_x = \int_A y^2 {\rm d}A$
(This is an important quantity in strength of materials/Euler-Bernoulli beam theory.)

What the heck does Moment mean though? So before it was torque, now it is something associated with area? Then there is moment of inertia, I am confused with the repeated use of the word moment. Does it have some sort of general meaning?

jhae2.718
Gold Member
Moment can be many things. In engineering we often use moment and torque interchangeably.

Moment of inertia can also have multiple meanings: it can be a measure of how mass is distributed about an axis (mass moment of inertia; used in dynamics) or a measure of how area is distributed about an axis (in strength of materials). This latter definition is also known as the Second Moment of Area.

You can think of moments as being quantities measured about an axis or surface. More abstractly, in mathematics a moment is a measure about a set of points (this is ultimately where the term comes from). The multiplicity of meaning of the term "moment" can get confusing when you're first exposed to it.

SteamKing
Staff Emeritus