# What is the formula for the first moment?

1. Apr 16, 2015

### avito009

The first moment divided by the total mass is the center of mass. Now I know that a moment is a combination of a physical quantity and distance. So what is the First moment made up of? I know that moment of inertia is made up of mass times the radius (distance) squared.

2. Apr 16, 2015

### Staff: Mentor

The first moment of mass is just mass times distance.

3. Apr 22, 2015

### avito009

So is the second moment of mass "Moment of Inertia"?

4. Apr 22, 2015

### Staff: Mentor

Indeed it is.

5. Apr 23, 2015

### avito009

According to Wikipedia, the first moment divided by the total mass is the center of mass. But as per what you say the first moment is mass times distance so if we divide that by the total mass we get the answer as distance. m*r/m=r. But distance is not the same as centre of mass. How?

6. Apr 23, 2015

### Staff: Mentor

That will give you the distance of the center of mass from some reference point (where the distances were measured from).

More precisely, those distances really should be position vectors. To get the position vector of the center of mass, you would add up all the first moment elements ($\Sigma m_i \vec{r}_i$) and then divide by the total mass.