Units of Inertia and Moment of Inertia

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Discussion Overview

The discussion revolves around the concepts of inertia and moment of inertia, exploring their definitions, units, and relationships. Participants examine whether inertia is a quantity or quality phenomenon, the nature of moments and torque, and the similarities between linear and rotational motion equations.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that inertia is a property that can be quantified, with the SI unit being kilogram (kg).
  • Others propose that inertia can be influenced by factors beyond mass, such as electromagnetic energy.
  • There is a discussion about whether inertia and mass are the same, with some stating they are closely related but not identical.
  • Participants clarify that moment refers to the distance from a force to a reference point, and torque is described as the moment of force.
  • One participant notes the similarity between linear and rotational motion equations, drawing parallels between force and torque.
  • Some participants express uncertainty about the implications of reversing the direction of rotation in a pump and its effects on flow direction and amount.

Areas of Agreement / Disagreement

Participants generally agree on the unit of moment of inertia being kg.m² and that inertia is measured in kg. However, there is no consensus on whether inertia and mass are the same, and the discussion includes multiple competing views on the nature of inertia and its implications.

Contextual Notes

Some statements about the definitions and relationships between inertia, mass, moment, and torque remain unresolved, with participants expressing differing interpretations and understandings.

Who May Find This Useful

Readers interested in the foundational concepts of physics, particularly in mechanics, may find this discussion relevant.

Mikealvarado100
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Hi
Unit of 'moment of inertia' is kg.m^2, Which I know. But about Inertia itself, is it a quantity phenomenon or quality phenomenon? What is it's unit?
 
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Inertia is the resistance of an object to be affected by a force. It can be caused by mass, but e.g. EM energy can have this effect too. See here!("Feynman lectures on physics" vol.2 has a chapter about this too.)
Moment of inertia is the resistance of an object to a torque. Its called moment of inertia because of the way it is calculated. See here and here!
 
Inertia is a property defined both as a general quality (what it is), and also in the particular sense (how much you have of it), in which case -- i.e. when you want to put a number to it -- kilogram (kg) is the SI unit.
 
Let me explain what I understood from your explanation and references mentioned. Please correct if you find mistake:
1- Inertia is a quantitative and it's unit is 'Kg'. Am I right? Do Inertia and Mass have same unit? Does it mean that both are the same?
2- Moment is a CONCEPT. And it refers to DISTANCE of a subject (like force) and the point which that subject (e.g. force) is applied to that subject. Yes?
3- Torque is 'moment of Force' which is shortened to 'moment'.
 
Mikealvarado100 said:
Unit of 'moment of inertia' is kg.m^2, Which I know. But about Inertia itself, is it a quantity phenomenon or quality phenomenon? What is it's unit?
'Moment of inertia' is for rotational motion what 'mass' is for linear motion.
 
Mikealvarado100 said:
Inertia is a quantitative and its unit is 'Kg'. Am I right?

Correct. The SI unit of measure for inertia is kilogram (symbol kg -- both k and g are lower case). Kilogram is one of the base units in the SI.

Mikealvarado100 said:
Moment is a CONCEPT. And it refers to DISTANCE of a subject (like force) and the point which that subject (e.g. force) is applied to that subject. Yes?

Moment is both an abstract concept and a physical reality. I would substitute the term physical quantity for subject -- and, incidentally, this physical quantity may be a scalar or a vector (force, mass, area, electric charge, etc).

Yes, moment does refer to the distance from the quantity to the reference point (or the perpendicular distance to the reference axis). I call this distance the moment arm. You have to multiply the quantity in question by the moment arm in order to get the moment. (It's a cross product but ordinary algebraic multiplication works in simple cases.) Moments are classified according to the power to which the moment arm is raised (usually the first or second power).

Mikealvarado100 said:
Torque is 'moment of Force' which is shortened to 'moment'.

Right. Torque is the first moment of force, and it's a vector.
 
Last edited:
In that thread you appear to be considering a machine that can function as a pump (power is absorbed) or a turbine (power is produced), correct?

Mikealvarado100 said:
1- Inertia is a quantitative and it's unit is 'kg'. Am I right? Do Inertia and Mass have same unit? Does it mean that both are the same?

They're closely related but technically not the same thing. Mass in all cases is the quantity of matter. But mass can mean the hypothetical ability to be pulled by gravity, or it can refer to resistance to acceleration (inertia).
 
There is a similarity between the equations of linear and rotational motion...eg...

Force = mass * acceleration
Torque = moment of inertia * angular acceleration

The linear equations of motion (SUVAT) can be similarly modified.
 
  • #10
David Lewis said:
In that thread you appear to be considering a machine that can function as a pump (power is absorbed) or a turbine (power is produced), correct?

Dear David
Yes, I am talking about a pump which can work as a turbine so. My exact question there is, when impeller rotates inversely, what happens? Direction of rotation affect on amount of flow? direction of flow? or both of them?
Please write your idea there.
 
  • #11
When the direction of rotation of the impeller reverses then the flow is also reversed. That is, fluid enters the pump's discharge outlet and comes out through the suction inlet. In this situation, mechanical power is available at the impeller shaft.

The advantages of operating a pump as a turbine are convenience, simplicity and low cost. The disadvantages are low efficiency and lack of versatility.
 

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