The result would have units of kg*m^4.

Click For Summary

Discussion Overview

The discussion revolves around calculating the total moment of inertia of an electropump, which consists of a motor and a pump, each with their own moment of inertia. Participants explore whether the total moment of inertia should be calculated by summing (A + B) or multiplying (A * B) the individual moments of inertia. The conversation also touches on the definition and units of moment of inertia.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants inquire whether the total moment of inertia of the electropump is calculated by summing the individual moments (A + B) or by multiplying them (A * B).
  • One participant clarifies that both components are fixed to the same shaft and rotate at the same rate, suggesting that the calculation relates to how the shaft will accelerate under torque.
  • There is a discussion about the definition of moment of inertia, with references to its dependence on mass distribution and the rotational axis.
  • Participants debate the correct units of moment of inertia, with some asserting it is 'kg*m^2' rather than 'kg*r^2'.
  • One participant poses a question about the units resulting from multiplying two quantities expressed in 'kg*m^2'.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to calculate the total moment of inertia, as there are competing views regarding whether to sum or multiply the individual moments. Additionally, there is disagreement on the correct units of moment of inertia.

Contextual Notes

There are unresolved assumptions regarding the specific configurations of the electropump and the definitions being used for moment of inertia. The discussion also reflects varying interpretations of units in the context of physics.

Mikealvarado100
Messages
52
Reaction score
0
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?
 
Physics news on Phys.org
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?
 
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?
 
Mikealvarado100 said:
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?
What is the definition of moment of inertia?

Or... what are its units?
 
  • Like
Likes   Reactions: CWatters
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.
 
Mikealvarado100 said:
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.
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:
  • Like
Likes   Reactions: CWatters

Similar threads

Undergrad Calculating Mass Inertia Product - Examples 1 & 2
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad How find moment of inertia for car in turn?
  • · Replies 49 ·
2
Replies
49
Views
6K
Undergrad Moment of inertia of human body with limbs at an angle
  • · Replies 12 ·
Replies
12
Views
2K
Undergrad Counter-rotating mass to counteract inertial spin
  • · Replies 10 ·
Replies
10
Views
3K
Undergrad How Is Power Calculated in a Rotating System with Variable Speed?
  • · Replies 7 ·
Replies
7
Views
2K
Graduate How can I calculate the moment of inertia of any object?
  • · Replies 10 ·
Replies
10
Views
3K
Undergrad Is it possible to create linear acceleration using nothing but different moments of inertia beteen a wheel and axle?
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad Addition of moments of inertia
  • · Replies 35 ·
2
Replies
35
Views
7K
Undergrad I'm calculating more energy out than I put in
  • · Replies 138 ·
5
Replies
138
Views
9K
Undergrad What is the Relationship Between Inertia and Moment of Inertia?
  • · Replies 3 ·
Replies
3
Views
2K