# Is there a named unit for inertia?

• synch
In summary, the concept of mass is synonymous with the concept of inertia, which follows directly from Newton's 2nd law.
synch
That is, an honorific title. Eg force is cited in Newtons, charge in Coulombs, etc, as opposed to kilograms.. Generically "m" seems to be used as a symbol I guess but it overlaps with generic references to mass.

eg

F = ma

is usually taken as referring to mass per se not inertia. But it would make a lot more sense to me, if it was actually referring to inertia instead. One could define a unit of inertia, as a sort of inverse Newton, and use that instead eg if Z is one inverse Newton it leads to

F = Za

where a is the acceleration still.

The SI unit of inertia is the kilogram

vanhees71
I am wondering if the subjective "mass" experienced in everyday life is actually always inertia. It makes some sense, if you think that a weight is a mass being accelerated but restrained by environment. (?) eg a freefalling apple is apparently weightless - but it still has inertia (eg sideways) (??)

Mustofa Khalid Ovi
To reword it a little, I am coming to the view that the force required to (eg) support a weight is not simply an opposing force to balance the gravitational force on a mass, but rather the force required to constantly (de-) accelerate the mass back from the effect of the gravitational acceleration. Probably I need to think about momentum instead of/as well as/ inertia.

synch said:
the force required to (eg) support a weight is not simply an opposing force to balance the gravitational force on a mass, but rather the force required to constantly (de-) accelerate the mass back from the effect of the gravitational acceleration
This seems like a distinction without a difference. Either way the math would be the same.

Sync: How about the amount of energy needed to heat an object? or the amount of hardener needed to set 1g of epoxy resin? Do these things depend on mass or inertia :-)

CWatters said:
Sync: How about the amount of energy needed to heat an object? or the amount of hardener needed to set 1g of epoxy resin? Do these things depend on mass or inertia :-)
Those depend on amount of matter (moles). It so happens that every molecule has the same inertia making that amount to be directly related to the unit of inertia (kg).

Inertia is the name of a concept, not the name of a quantity.

Khashishi said:
Inertia is the name of a concept, not the name of a quantity.
The formula F=ma says that a block of mass 1 kg has an inertia (it resists movement) such that it takes a force of 1 Newton to give it an acceleration of 1 m/s2.
In other words, mass is synonymous to inertia, which follows directly from Newton's 2nd law.

xAxis
I guess I am suggesting that observations on the phenomenon "mass" are usually actually observing inertia instead. Although the numbers are the same, and the two are often used synonymously, inertia is often said to be a property of mass - but I am suggesting, inertia is the actual phenomenon underlying the events observed, and the various laws should reflect that.

(edit) What happens in the case of photons ? Having no mass they should not classically have any inertia - but the same logic suggests they should not have momentum either, which is found to be incorrect. Does energy itself have inertia ? I am wondering if the inertia observed for a mass , is actually an artefact of the mass-equivalent energy ?

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How would any laws change?

FactChecker
It sounds like that would just be a name change from "mass" to "inertia". If that is true, then the issue depends on established perceptions of what each term means. When I think of "inertia", I only think in terms of resistance to the change of motion, both linear and rotational. When I think of "mass", I think of much more -- energy, gravitational attraction, distortion of space-time, subatomic particles, chemical formulas, etc. in addition to momentum,

I don't see any advantage to changing the name and I see many disadvantages.

synch said:
I am suggesting, inertia is the actual phenomenon underlying the events observed, and the various laws should reflect that.
How would any of the laws be altered to reflect that? So far you suggested F=Za, which clearly leads to Z=m. Again, this is a distinction without a difference. You can change the word from "mass" to "inertia" or to "flubnubitz", it doesn't change any of the laws.

FactChecker
I did not suggest the laws should be changed. I do however think the concepts of mass and inertia are different, and that force acts against the inertia of a mass instead of the mass itself. The points about mass referring to quantity etc are very sensible in real life ! I would hate to argue that mass is not actually present , especially as a former chemist well remembering gravimetric analysis, using precision balances etc. It is kind of confronting, every human on the planet experiences gravity and mass at a foundation level before even walking . Certainly at a gut level, when I think of mass, I see a large 5 ton weight from Acme Ltd, made of cast iron, usually suspended above a so-unlucky coyote...like most I guess.

synch said:
I did not suggest the laws should be changed.
Then it is a purely semantic argument you are making. Call it what you will, it makes no difference to the physics. (Although it will make communication more difficult, for no particular benefit)

russ_watters
synch said:
I do however think the concepts of mass and inertia are different, and that force acts against the inertia of a mass instead of the mass itself.
Ok, so what do we do with that? Create a new unit for "inertia"? Add a conversion factor of 1 flubnubitz/g into equations where inertia is used? I don't see what value/clarity this adds.

Let's try another example: specific heat capacity in J/kg-k. It's different for every substance, so we need the conversion constant when we use it: e=cmT. But if it were the same for each element, we would just be left with the conversion equation of e=mT (J=kg*K), which works very much like f=ma.

It's fine to say that mass is the amount of matter and heat capacity and inertia are properties of matter, but you don't have to add anything to the equations to say that: the equations already say it!

synch said:
I am suggesting, inertia is the actual phenomenon underlying the events observed, and the various laws should reflect that.
synch said:
I did not suggest the laws should be changed.
Heh?

russ_watters and Dale
It does not change the maths or calculated amounts - but it does change the comprehension. of the physics. It is not worth changing the laws in a practical sense, but the use of inertia and mass interchangeably is perhaps an imperfection. To suggest a change would be like airbrushing the Sistine Chapel !
In any case I present - not entirely seriously - some possible honorific units

Inverse Newton : a Notwen.
Inverse kilogram : a Margolik
and an inverse Planck : a Kcnalp. (!)

synch said:
It does not change the maths or calculated amounts - but it does change the comprehension.
I am opposed to change for change's sake. I see no benefit to this

synch said:
the use of inertia and mass interchangeably is perhaps an imperfection.
Your proposal doesn't "fix" that at all. You still have Z=m. All your proposal does is change names for the same concept

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## 1. What is inertia and why is it important?

Inertia is an object's resistance to change in its state of motion. It is important because it helps us understand how objects move and interact with each other.

## 2. Is there a unit of measurement for inertia?

Yes, the unit for inertia is kilogram-meter squared (kg·m²) in the International System of Units (SI). It is also commonly measured in pound-feet squared (lb·ft²) in the imperial system.

## 3. Can inertia be measured directly?

No, inertia cannot be measured directly. It is a property of an object and can only be calculated using other known quantities, such as mass and velocity.

## 4. How does inertia relate to Newton's first law of motion?

Inertia is directly related to Newton's first law of motion, which states that an object at rest will stay at rest and an object in motion will stay in motion with constant velocity unless acted upon by an external force. Inertia is the reason why objects tend to continue their motion or stay at rest unless a force acts upon them.

## 5. Are there any other related units for inertia?

Yes, there are other units that are related to inertia, such as the moment of inertia, which is a measure of an object's resistance to changes in its rotation. This is measured in units of kilogram-meter squared (kg·m²) or pound-feet squared (lb·ft²) depending on the system of measurement. Additionally, the concept of inertia is also closely related to the concept of mass, which is measured in kilograms (kg) or pounds (lb).

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