B Looking for help Understanding Inertia

[Physics_is_beautiful]

TL;DR Summary

questions on inertia

Just a thread to see how inertia works, just studied the laws of motion, and have been fascinated with it.. could use some help on it tho :)

 

[PeroK]

Inertia isn't really part of my physics vocabulary, other than as another name for mass.

 

[berkeman]

TL;DR Summary: questions on inertia

Just a thread to see how inertia works, just studied the laws of motion, and have been fascinated with it.. could use some help on it tho :)

As I mentioned in your New Member Introduction thread, it is best for you to post links to the reading you've been doing so far. Please do that so we can do a better job of helping you. Thanks.

 

[DaveE]

It's hard for us to help if you don't ask a question. We don't know what's confusing you.
Here are some good short lectures about Newton's laws.
https://www.khanacademy.org/science...on-s-first-law/v/newton-s-first-law-of-motion

 

[Physics_is_beautiful]

To clarify, I have been particularly puzzled by what causes inertia,
ps - thanks for the links.

It's hard for us to help if you don't ask a question. We don't know what's confusing you.
Here are some good short lectures about Newton's laws.
https://www.khanacademy.org/science...on-s-first-law/v/newton-s-first-law-of-motion

 

[PeroK]

To clarify, I have been particularly puzzled by what causes inertia,
ps - thanks for the links.

It depends what you mean by "cause"? Matter generally has the properties of mass and electric charge. Those lead to the two fundamental forces of classical physics: gravity and electromagnetism. Mass is also the property that determines how fast a body accelerates under a given force. See Newton's second law: $F = ma$.

To understand why elementary particles have mass and electric charge, you need to delve deep into quantum mechanics.

 

[vanhees71]

Inertia isn't really part of my physics vocabulary, other than as another name for mass.

In this sense it's also part of my physics vocabulary, but only when I'm within the Newtonian world view :-).

 

[Herman Trivilino]

To clarify, I have been particularly puzzled by what causes inertia,

It's the fact that states of uniform motion can't be distinguished from one another.

 

[PeroK]

It's the fact that states of uniform motion can't be distinguished from one another.

You might have to expand on that!

 

[Sailor Al]

To clarify, I have been particularly puzzled by what causes inertia,
ps - thanks for the links.

I think you need to see that inertia is not a property of classical physics. The best you will find is an obscure reference like it's the tendency of an object to resist a change in momentum. But really it's best not to dwell on it. You won't find inertia defined in reputable physics textbooks (and don't get me started on Wikipedia!)
If you mean mass, than, mass is the term to use. If you mean momentum, then use momentum.
Inertia really isn't a useful concept in physics.

 

[jbriggs444]

I think you need to see that inertia is not a property of classical physics. The best you will find is an obscure reference like it's the tendency of an object to resist a change in momentum. But really it's best not to dwell on it. You won't find inertia defined in reputable physics textbooks (and don't get me started on Wikipedia!)
If you mean mass, than, mass is the term to use. If you mean momentum, then use momentum.
Inertia really isn't a useful concept in physics.

Inertia is a feature of classical physics named "mass". Or, to be more precise, "inertial mass".

Free objects have no ability to resist changes in momentum. If you apply a force to them, they will gain momentum at a rate proportional to the applied force. In fact, "force" is defined as a rate of transfer of momentum. With a coherent system of units, momentum changes at a rate equal to the applied force, not just proportional.

If you want an object to resist changes in momentum, it is easy to accomplish. You nail it down, screw it in place and brace it securely. Now any applied force will be resisted by the support structure, reducing the change in momentum.

Reducing it on the target object anyway. Momentum is conserved. If a force is applied, that momentum transfer will manifest somewhere. If you have a truly external force and you push on a box nailed to the Earth, the Earth will move in response. If you, standing on the kitchen floor, are pushing a box that is nailed to the kitchen floor, you'll get the same result as lifting on your own bootstraps.

 

[Frabjous]

Inertia is a feature of classical physics named "mass". Or, to be more precise, "inertial mass".

Free objects have no ability to resist changes in momentum. If you apply a force to them, they will gain momentum at a rate proportional to the applied force. In fact, "force" is defined as a rate of transfer of momentum. With a coherent system of units, momentum changes at a rate equal to the applied force, not just proportional.

If you want an object to resist changes in momentum, it is easy to accomplish. You nail it down, screw it in place and brace it securely. Now any applied force will be resisted by the support structure, reducing the change in momentum.

Reducing it on the target object anyway. Momentum is conserved. If a force is applied, that momentum transfer will manifest somewhere. If you have a truly external force and you push on a box nailed to the Earth, the Earth will move in response. If you, standing on the kitchen floor, are pushing a box that is nailed to the kitchen floor, you'll get the same result as lifting on your own bootstraps.

What do accepted theories say about the equivalence of gravitational and inertial mass or is it an experimental observation?

 

[jbriggs444]

What do accepted theories say about equivalence of gravitational and inertial mass or is it an experimental observation?

Google for the equivalence principle which states that they are identical.

Of course, the match between the two conveniently arises from the experimental observation that objects fall at the same rate and is implicit in Newton's law of gravitation.

 

[jbriggs444]

This Wiki article is clear enough.

In classical Newtonian mechanics, there are two flavors of gravitational mass: active gravitational mass and passive gravitational mass. In Newton's gravity formula:$$F=G\frac{mM}{r^2}$$the mass of the object doing the attracting is $M$ and is the "active gravitational mass". The mass of the object being attracted is $m$ and is the "passive gravitational mass".

While there is a conceptual distinction that can be made between "inertial mass", "active gravitational mass" and "passive gravitational mass", it is normally assumed, in Newtonian mechanics at least, that all three are identical. Experiment can only say that they are proportional. Convention, quite reasonably, says that they are identical.

In General Relativity, the notion of "active gravitational mass" is replaced by the stress energy tensor and the equivalence of inertial mass and passive gravitational mass is baked into the theory. In our everyday environment, stress is negligible and the primary source of energy density is mass density, so the "stress energy tensor" and "active gravitational mass [density]" are more or less the same thing.

A Wiki reference is fine for this since it matches what @Frabjous meant by the term, what I understood the term to mean and what essentially any physicist or physics student would understand the term to mean. There is no controversy here.

 

[berkeman]

Thread is locked temporarily for Moderation. Lordy.