# I About 'Moment of Inertia'

1. Jul 10, 2016

Hi
Assume a Electropump which motor's moment of inertia is A and pump's moment of inertia is B.
How can calculate Electropump's moment of inertia? Is it A+B or A*B?

2. Jul 10, 2016

### jbriggs444

These are both fixed to the same shaft so that they both rotate at the same rate? And you are trying to use this to figure out how the shaft will accelerate under torque?

3. Jul 10, 2016

jbri...
Yes, Both are combined as an Electromotor. Both moment of inertia is not specified, but each one has a specified moment of inertia. How is total moment of inertia calculated? A+B or A*B?

4. Jul 10, 2016

### jbriggs444

What is the definition of moment of inertia?

Or... what are its units?

5. Jul 11, 2016

It is called also 'angular mass' or 'rotational inertia'. Wikipedia defined it as ''torque needed for a desired angular acceleration about a rotational axis. It depends on the body's mass distribution and the axis chosen, with larger moments requiring more torque to change the body's rotation'
It's unit is 'kg*r^2' which Kg is the distance between mass and rotational axis.

6. Jul 12, 2016

### jbriggs444

What I was getting at was its definition in terms of how you would calculate the moment of inertia of an object - i.e. as the sum of the mass of each small part times the square of that part's distance from the axis of rotation.

Its unit is not kg*r^2. It is kg*m^2. You always put standard units (seconds, meters, kilograms, amperes, etc) into the units. You do not put measured quantities (half-life, radius, mass or current) in there.

Now, if you multiply a quantity expressed in kg*m2 by another quantity expressed in kg*m2, what units would the result have?

Last edited: Jul 12, 2016