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Talk:Inertia

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Inertial frame[edit]

If inertia is the same thing as mass, why don't we never call such a frame, a 'frame of mass', or 'massive frame'? --24.202.163.194 20:59, 31 December 2005 (UTC)[]

(Answer: Inertia is not the same thing as mass. Inertia is a property of mass, and is the resistance of the mass to acceleration.) — Preceding unsigned comment added by Zee99 (talk • contribs) 00:00, 7 December 2013 (UTC)[]

"However, in frames which are experiencing acceleration (non-inertial frames), objects appear to be affected by fictitious forces. For example, if the railway carriage was accelerating, the ball would not fall vertically within the carriage but would appear to an observer to be deflected because the carriage and the ball would not be traveling at the same speed while the ball was falling."


My Q.: Here , is the observer being referred to , an observer inside the carriage or an observer outside the carriage? —Preceding unsigned comment added by Geetrana (talk • contribs) 05:07, 15 November 2009 (UTC)[]

Inside. Archelon (talk) 19:20, 14 April 2015 (UTC)[]

Inertia is a force[edit]

The article seems to be missing the interpretation of Newton that inertia is a force that only appears during acceleration, as the equal and opposite complement to the accelerating force. This interpretation needs to be added to the article but I'm not sure where. Any ideas? Also, can anyone point me to an online source for full text arguments for comparative interpretations of Newton's writings, including the Latin, and including a literal translation (word for word, not interpreted phrases) of his three laws of motion? How about the best library reference book you know on comparative interpretations? My searches find sites where the "inertia is a force" concept is used (such as sense 1. here), but no sites linking the translation text. -- Another Stickler (talk) 00:01, 3 December 2008 (UTC)[]

Reading more, I see inertia seems to have aquired (at least) three main interpretations historically:
1) A principle that describes how things behave, in other words a law of repeated observation that unaccelerated bodies will continue in a straight line at a fixed speed (including speed zero).
2) A measurable property of matter equivalent to mass and remaining constant during acceleration and non-acceleration.
3) A force reacting to or opposing the net applied force during acceleration, increasing exactly to match any accelerating force, and disappearing otherwise.
Perhaps the complication of multiple interpretations arose because Newton's language was not consistent or because his words became ambiguous in translation or because others had their own ideas they wanted to read into it. However it happened, even though Newton stated that forces are always in matched pairs, implying interpretation three above that inertia is a force (because an applied accelerating force must have an equal-and-opposite matching reaction force), that interpretation was not established to the extent that it left a hole for D'Alembert (who was 10 when Newton died) to introduce a "new force" called the "force of inertia" defined as "the negative of the product of mass times acceleration" [1]. My point is that Newton already said that inertia is a force; D'Alembert simply paraphrased Newton, as Newton paraphrased some ideas of others including Galileo (who died the year before Newton was born). I still think this article needs to include interpretation three, but I have to do more research before carving it into the text. Can anyone help? Do any historians know who else beside Newton and D'Alembert wrote that inertia is a force? -- Another Stickler (talk) 22:46, 7 January 2009 (UTC)[]

I found a modern book with readable text that treats inertia as a force, In the Grip of the Distant Universe, By Peter Graneau, Neal Graneau. Here's one passage, "If this force did not exist, then any applied force would produce an infinite acceleration and the universe would have collapsed long ago due to the force of gravity." (page 19) -- Another Stickler (talk) 22:34, 27 January 2009 (UTC)[]

I disagree that inertia is a force. (I also disagree with the popular comment that inertia is measured in kilograms or lbm.) Inertia is a principle. Newton's first law of motion is often referred to as the principle of inertia.
It is mass that is measured in kilograms or lbm. Mass and inertia are different. It is force that is measured in newtons or lfb. Force and inertia are different.
The principle of inertia was most significant when it was first explained by Newton because up until that time the universal view was that if a body was to continue moving at a constant speed it required a constant force applied to it, even if the body was moving in a constant direction. Newton exposed that universal view as being in error and he did so with his principle of inertia. This principle is equal applicable to an elephant and a gnat. The elephant has greater mass than a gnat but it would be incorrect to say an elephant has more inertia than a gnat.
Inertia is a principle. Inertia is not force or mass so it is not measured in newtons or kilograms or pounds. Dolphin51 (talk) 01:44, 11 January 2010 (UTC)[]
If I understand the wikipedia concept correctly, we're not here to weigh the relative merits of conflicting interpretations, as tempting as that is; there are other forums for that; we're here to make sure the article reflects the literature. I already noted above (see interpretation 1) that inertia has been called a principle. The truth is, the other two interpretations (property of matter, and force) are found in the literature as well, and the article is incomplete while it doesn't include them. Another Stickler (talk) 19:08, 9 February 2010 (UTC)[]
I wholeheartedly agree that it isn't the role of Wikipedia to arbitrate on competing explanations. Where the body of authoritative literature on a subject contains two or more explanations that are different it is the role of Wikipedia to present all those differing explanations. The only constraint on Wikipedia is that the various explanations must be adequately supported by references and in-line citations to allow independent verification that the different explanations are in fact to be found in authoritative literature on the subject.
Inertia is poorly supported by references and in-line citations. Most of the Notes relate to the history and development of the subject. A number of the statements made about inertia are conspicuously lacking an in-line citation. Therefore they are likely to be nothing more than someone's original research.
For example, the second sentence says [Inertia] is represented numerically by an object's mass. No in-line citation has been provided to support this statement. I suspect that whenever an editor goes in search of an authoritative document to use as an in-line citation for this statement that editor will find nothing. (Consider a mosquito, and an elephant with a mass ten thousand times greater than that of the mosquito. When the resultant force on both animals is zero the principle of inertia predicts that their velocities will be constant. The velocity of the elephant won't be ten thousand times more constant than that of the mosquito. So it isn't true to state that inertia is represented numerically by the object's mass, and it will be very difficult, perhaps impossible, to find an authoritative document that supports the statement. It is true that a body's mass is a measure of its resistance to acceleration under the influence of a resultant force. But acceleration in response to a force is the subject of Newton's Second Law of Motion, which is not the principle of inertia.)
Perhaps the way forward with this article is to obtain more in-line citations to support what is already there. Any notions that can't be supported by in-line citations should be deleted until suitable in-line citations can be found. When the quality of the article has been raised in this way it would be reasonable to add alternative explanations of inertia, supporting them by suitable in-line citations. Dolphin51 (talk) 22:23, 9 February 2010 (UTC)[]
Beer & Johnston's "Vector Mechanics for Engineers" cites D'Alembert and talks about "inertial forces". This is a classic textbook and cannot be ignored. Besides, in college I never solved a single problem with inertia in kilograms, it was always in newtons. Having a "principle" may be nice for philosophical discussions, but you'd never solve any engineering problem with just that. Aldo L (talk) 21:44, 10 August 2010 (UTC)[]
The expression inertial force is commonly used - it refers to a particular kind of force, not a particular kind of inertia. (In this expression, inertial is merely the adjective, not the noun.)
Aldo L has written that when he was in college he solved problems with inertia in newtons. That is incorrect - when he used newtons he was referring to forces. When he used kilograms he was referring to masses.
The word inertia refers to Newton's Principle of Inertia, better known as Newton's First Law of Motion. Inertia is a principle, not a force or a mass. Dolphin (t) 23:22, 10 August 2010 (UTC)[]
So we don't know what inertia is, but we know that it's related to both the mass and the linear velocity of a material object. And these 2 values commute to be the linear momentum of the object. And what will happen if an object is object is translating perpendicularly to the direction of connection of a restraining string? Well, neglecting the mass of the restraining string, we can say that the result will be that string will be stretched and subject to a tensile force, which it can resist and create a tensile stress in the string as well as a side force on the line of motion of the object.WFPM (talk) 21:19, 17 February 2011 (UTC)[]
Inertia is not evident only in linear accelerations. An angular acceleration requires a torque. The angular form of Newton's first law of motion states that if the resultant torque on a body is zero, the body will not undergo an angular acceleration. Newton's first law is often described as the Principle of Inertia and it is equally applicable to forces and torques. Dolphin (t) 21:33, 17 February 2011 (UTC)[]
Thank you! We're discussing about whether a centripetal restraining force in a string can create an increasing stress in the string. See String trimmer. I said no, and now I'm trying to figure it out. Your initial input sounds like no, because I don't see any torque. And I was about to try to integrate the stress in the string over a distance to see if increasingly added up. Too bad I don't know more mathematics.WFPM (talk) 22:10, 17 February 2011 (UTC) Oh excuse me, because the discussion about the string tension was the last item of a Talk:Centrifugal force discussion. Se la vie.WFPM (talk) 22:18, 17 February 2011 (UTC)[]

Shouldn't all formulas have notation that distinguishes between vector & scalar quantities, either setting vector symbols in bold/italic, or with arrows or super-script lines? I'd change this myself but see that the original have "math" attribute written beside them, that I'm guessing is a plain text attribute, not a link to a math editing script? (Emperor Zhark, 9 February, 2013) — Preceding unsigned comment added by Emperor Zhark (talk • contribs) 15:39, 9 February 2013 (UTC)[]

Excuse me for the 8 years delay to clarify the things. I've added some changes and complements which show that Dolphin51 view about inertia is incorrect! Inertia is indeed a force, which Newton called "vis inertiae". I give much more details on this subject on my paper "Revisiting Newton's vis inertiae", to appear in 'Am. J. Phys.' this year (2021). FabioMSL (talk) 02:43, 26 July 2021 (UTC)[]

@FabioMSL: Wikipedia has two articles on this subject - Inertia and Inertial force. Clearly, an inertial force is a force, but within Wikipedia's coverage of this subject it is doubtful that inertia is also a force. Dolphin (t) 06:17, 26 July 2021 (UTC)[]

Caption[edit]

Why does the caption read "Well, so far we've looked at the seat belts you use in your car when your doing 40 miles an hour"? Can't NASSA write? --61.69.3.155 (talk) 06:07, 7 February 2009 (UTC)[]

I also would like to quote:

"... It talks about bodies in rest and bodies in motion. But were (sic) not talking about human bodies."

There may be no reasonable way to fix this, but this seems to be quite an annoying issue. Any ideas? --JukeJohn (talk) 01:25, 5 March 2009 (UTC)[]

"Invention" of Relativity[edit]

This question is more generic than this particular article. In section "Mass and Inertia" there is the sentence "when the theory of relativity was not yet created". Is this the proper way to refer to it? The physics that the Theory of Relativity expresses were not created, but the theory itself was created. While the sentence is grammatically correct, it still feels wrong to say. Shouldn't it say something more like "prior to the formalization of TOR" or "when the TOR had not yet been expressed." -- The current phrase feels a bit like crediting Ben Franklin for "inventing" lightning. B-Con (talk) 00:31, 27 April 2009 (UTC)[]

Hmm... I see what you mean. As you say, the sentence is essentially correct as written, but I can see how confusion could arise. And I'm not sure that it's accurate to say that a theory is "created", anyway. "Developed" might be a better word or, as you suggest, "formalized" or "expressed". To be honest, none of those feels just right, but I can't think of any others. Of the three, "formalized" would be the most proper for an audience primarily composed of scientists but I think "expressed" might be the best one for Wikipedia. I'm not going to change it just yet though. I'd rather have another opinion or two first, and maybe even some more proposed words. -- edi(talk) 01:18, 27 April 2009 (UTC)[]
I agree, it's hard to find the right word. I pondered it for a while as well before posting. I also agree on the use of formalized/expressed. I was going to make an edit, but figured I should ask to see if there exists standard protocol for this type of reference. It's surely an issue that comes up often and I'm sure (hope?) someone has an elegant -- or at least non-awkward -- method of dealing with it. I'll try to ask a math professor tomorrow. -- B-Con (talk) 05:40, 27 April 2009 (UTC)[]

NASA has been removed[edit]

I strongly suggest that this NASA video is removed immediately from the page. In one scene (at 1:26) there is the claim that "... because we are moving faster, we have a lot more inertia ...". This is of course a serious misconception and very misleading. That video is a shame for NASA. -- CHRV (talk) 23:04, 16 November 2009 (UTC)[]

Just FYI: I removed the video some time ago. -- CHRV (talk) 22:36, 4 February 2010 (UTC)[]

Media needed[edit]

This article requires some media. A couple of explanatory pictures would be great. It's current format looks a bit dull.Ravi84m (talk) —Preceding undated comment added 23:16, 6 April 2010 (UTC).[]

Last paragraph of Relativity section - isn't that GR rather then SR?[edit]

"Another profound, perhaps the most well-known, conclusion of the theory of Special Relativity was that energy and mass are not separate things, but are, in fact, interchangeable. This new relationship, however, also carried with it new implications for the concept of inertia. The logical conclusion of Special Relativity was that if mass exhibits the principle of inertia, then inertia must also apply to energy as well. This theory, and subsequent experiments confirming some of its conclusions, have also served to radically expand the definition of inertia in some contexts to apply to a much wider context including energy as well as matter."

I'm no physicist but isn't E = MC^2 General relativity? —Preceding unsigned comment added by 88.104.105.223 (talk) 12:39, 23 October 2010 (UTC)[]

I'm pretty sure that is in SR William M. Connolley (talk) 22:25, 24 April 2011 (UTC)[]

A novel perspective[edit]

I reverted [2] User:Kurtan, on the grounds that this isn't the place to be adding "novel perspectives". As far as I can tell, the Masreliez paper is exactly what it says - a novel perspective, not widely accepted or widely discussed. Which is to say, it looks fringe to me William M. Connolley (talk) 22:17, 24 April 2011 (UTC)[]

Another perspective[edit]

There is a simple experiment that can show that inertia has an extra component to it, than only mass. Imagine two one-ton hunks of steel. Shape one of them into a sphere, and the other one into a long rod (say 100 meters long). Suspend both (separately) so that they can swing like a pendulum (the long rod horizontal to the ground). Arrange to provide an impact-type of force to each mass (the same magnitude of force, of course). On the side of the sphere opposite to its impactor, suspend an ordinary steel spherical ball-bearing ball, with a 1-millimeter gap between the ball and the sphere. For the long rod, the impactor is set to apply force to one end of the rod. At the far end of the rod we suspend another identical small steel ball with another 1-millimeter gap. We can connect fine electric wires to all four suspended objects, and prepare a low voltage so that a circuit is completed when, say, the 1-ton sphere contacts the small bearing-ball. Now trigger the two impactors so that both 1-ton masses are struck simultaneously with equal force. The inertia of both objects must be overcome before they can move to hit/move the two suspended small steel balls. We will be able to accurately measure any difference in the time it takes the electric circuits to close.

If the two large masses have identical inertia, despite their hugely different shapes, then both circuits will close simultaneously. But in actual fact the sphere's ball will be contacted/moved first. See, when a force impacts a mass, there is a compression-wave that moves through the mass at the speed of sound (which in steel can be 5000 meters/second, often faster in harder steels). So, the far end of the 100-meter rod simply can't move at all, until that compression wave arrives (about 1/50th of a second) --while the compression wave in the sphere has much less distance to travel, so the whole sphere is more quickly affected by the applied force, than the long rod. Therefore it logically follows that the long rod and the sphere have two different magnitudes of inertia --"the thing that must be overcome before an object can move as a whole"-- despite the large objects in this experiment having identical masses. Which means, as initially stated, that the overall concept of inertia has an extra component to it (which mostly can be --and is-- ignored for ordinary everyday objects, but which should not be ignored if you really want to do accurate physics). V (talk) 22:09, 9 November 2011 (UTC)[]

Your "extra component" is merely a difference in velocity and the way that you are proposing to measure it. If, instead, you choose to measure the mass-average of the respective velocities then you find that both sphere and rod have the same speed at any instant. Dbfirs 07:58, 19 February 2013 (UTC)[]

Inertia proportional to mass[edit]

The lead section, second sentence, states: It is proportional to an object's mass. I can find nowhere in the article where a source is cited for this statement.

Mass is the numerical measure of the property (called inertia). Handbook of Physics, Condon and Odishaw, page 2-11. — Preceding unsigned comment added by Zee99 (talk • contribs) 19:10, 7 December 2013 (UTC)[]

I agree that the linear momentum of an object, and even the kinetic and potential energies of an object, are proportional to the object's mass, but I disagree about inertia. Inertia is not a quantity, it is a principle - Newton's First Law of Motion is often described as the Principle of Inertia. All objects, regardless of their mass, exhibit the principle of inertia - in the absence of a force they remain stationary or continue with uniform velocity.

Newton's Principle of Inertia tells us an elephant that is experiencing no resultant force remains stationary (or experiences an acceleration of zero.) It also tells us a mosquito that is experiencing no resultant force remains stationary (or experiences an acceleration of zero.) In what way are the responses of the elephant and the mosquito proportional to their masses?

Answer: Both the mosquito and elephant respond the same way when an equal force is applied. Imagine them to be in outer space. Now apply a force of one dyne to the elephant and one dyne to the mosquito. The elephant remains nearly at rest, but the mosquito is accelerated to a high speed. The difference is their inertial mass.

Unless someone can identify a reliable published source for the statement "inertia is proportional to mass", I propose we delete it. Dolphin (t) 01:53, 11 January 2012 (UTC)[]

Eight days have passed and no-one has identified a reliable published source, or objected to my proposal, so I have deleted the offending sentence. Dolphin (t) 05:20, 19 January 2012 (UTC)[]

I think the crux of that sentence is this: that the force required to cause an equal acceleration in each object is directly proportional to each objects mass (after disregarding friction and other forces). 183.90.103.161 (talk) 18:07, 18 February 2013 (UTC)[]

I have no idea where the offending statement was, so I cannot return it. The statement is correct if we modify it slightly, i.e. "It (inertia) is equal to an object's inertial mass, which is a function of speed." We need to always be careful to distinguish inertial mass from gravitational mass, and avoid simply saying "mass."

Inertial mass is normally taken to be the same as invariant mass which is identical to gravitational mass, so there is no confusion in just saying "mass". Relativistic mass is not used much because it depends on how fast the observer is travelling. Dbfirs 09:55, 8 December 2013 (UTC)[]

Using just the word "mass" confuses me because I know there's a difference between the two masses. I think that we'd be wrong in saying that "normally" it's used only one way because I've seen plenty of ways that it isn't used as invariant mass. To that end, we should explain what we're talking about and not assume the reader has this or that background. The term inertial mass is appropriate when we are referring to mass that has the speed range of zero to the speed of light, and inertial mass varies in quantity with speed. The gravitational mass doesn't vary with speed. And we can't say that inertial mass and gravitational mass are the same within an inertial reference frame unless we also specify that the reference frame is at rest. When the reference frame has speed, the inertial mass always exceeds the gravitational mass. Do you agree? — Preceding unsigned comment added by Zee99 (talk • contribs) 23:27, 8 December 2013 (UTC)[]

'The text states the following: In a given inertial frame, there is no measurable difference between gravitational mass and inertial mass. This sentence (used twice) makes no sense to me, and I'd appreciate help in understanding it. There is no measurable difference between inertial mass and gravitational mass at zero speed. At high speed (s ≈ c) there is a difference, but I don't know how to measure gravitational mass. The inertial mass is easy to measure. The part that is confusing me is the phrase "In a given inertial frame." Why is it needed? Why not delete it, and add "at zero speed" at the end of the sentence? — Preceding unsigned comment added by Zee99 (talk • contribs) 01:17, 9 December 2013 (UTC)[]

I was brought up with the idea that mass varies with speed, but modern physicists mean "invariant mass" when they talk about inertial mass. The concept of relativistic mass is just not used, though I agree that it appeared in texts more than 50 years ago. Speed is relative. There is no such thing as absolute speed, so how can we talk about "mass at a given speed"? In the inertial frame of the object being observed, inertial mass is indistinguishable from gravitational mass. Different observers moving at different speeds relative to the object will observe apparently different relativistic masses for the object, but always the same invariant inertial mass. Dbfirs 08:14, 9 December 2013 (UTC)[]

""mass at a given speed" means mass onboard an inertial reference frame which is observed by another inertial reference frame "at rest." Yes, there is a difference between the inertial mass on the moving frame and the gravitational mass. The gravitational mass on the moving frame is the same as the gravitational mass on the at-rest frame, but the inertial mass on the moving frame is larger than on the at-rest frame. Think of it this way: the gravitational mass deals with the quantity of gravitational field material which, unless you have something like a nuclear interaction, never changes. Thus the gravitational mass is constant at all speeds. However, in the process of acceleration, the gravitational field is distorted, making the inertial mass increase. When I computed the amount of distortion due to acceleration, it was exactly equal to that predicted by the Lorentz transformation for mass. And if you go to that place in space where the gravitational field is perfectly symmetrical, you will be at the absolute at-rest place, contrary to Einstein's assertion. If you'd like, we can continue this discussion off-line. — Preceding unsigned comment added by Zee99 (talk • contribs) 22:40, 9 December 2013 (UTC)[]

You seem to be unaware of the symmetry of relativistic observations. There is no such thing as an inertial reference frame "at rest" in an absolute sense. Mass in your "rest frame" will appear to be increased when observed from your "moving" frame, and thus will appear to be greater just because it is being observed from a moving frame. Dbfirs 19:59, 10 December 2013 (UTC)[]

Thank you for inserting the question at the Reference Desk, but I fear we are still not communicating. There are far too many issues being addressed here, and I'd like to examine each one, but let's continue with discussing the notion that gravitational mass and inertial mass are always equal. If I understand you correctly, you say they are and I say they aren't. I say that when a mass is "at rest", the two kinds of masses are equal. I say they are "at rest" when the isopotential surface of their gravitational field is a perfect sphere. When the mass is accelerated, the isopotential surface deforms, and the inertial mass increases. The gravitational mass remains a constant at all speeds. Yes, I'm familiar with the symmetry of relativistic observations, but again, I'm not sure you and I are speaking the same language. I differentiate between viewing frames by determining which one has been accelerated, and thus viewing from a frame that has not been accelerated is not the same as viewing from a frame that has been accelerated. That is, only the clock onboard an accelerated frame slows down. The clock's inertial mass has been increased during the acceleration, and that is the mechanism that causes the slowdown. The clock's gravitational field "has been effectively reduced" by being distorted during acceleration, but that is another topic I've been trying to avoid by claiming that the gravitational mass is unchanged at all speeds. Getting back to the clock. Suppose we have a clock "at rest" and we are onboard the accelerated frame wherein fields have been compressed. The symmetry in special relativity created a paradox by claiming a clock in either frame ("at rest" or accelerated) slows down when observed by the other frame. That was rescued by general relativity when it was demonstrated that only the accelerated clock slowed down. I explain it by examining the deformation of the gravitational field, and say that only the clock that was accelerated slows down because only its inertial mass is increased. Please forgive me, but I'm trying to clarify what I said, and I hope I haven't muddied the water.

Zee99,
  • It doesn't make sense to say that something is onboard (or in) an inertial reference frame. Everything is "in" every reference frame inasmuch as the reference frame assigns coordinates to it. A reference frame is just a way of assigning coordinates.

I think it makes perfect sense to me to say that a mass is in or "onboard" an inertial reference frame. What I thought we were addressing is whether, if the frame has been accelerated, does the inertial mass equal the gravitational mass. Personally, I don't have any trouble with addressing the two masses with or without an inertial reference frame.

  • There are various notions of mass, like rest mass, relativistic mass, and transverse/longitudinal mass, and they all have to work the same inertially and gravitationally. For example, ignoring gravity, a box of light has to have inertial rest mass even though a beam of light has no inertial rest mass. The inertial rest mass of the light in the box is the total inertial relativistic mass of the light with respect to the center-of-momentum frame of the light. The same has to be true gravitationally, since otherwise a box of light would violate the equivalence principle, so if you replace "inertial" with "gravitational" above it's still true.
  • There's nothing special about nuclear interactions. You can say that they convert mass to energy or that they don't, but it doesn't matter to gravity since mass and energy both gravitate. There's no way to change the amount of mass/energy that's visible to the gravitational force.

I'd love to discuss this more, but let's see if we can get agreement on whether inertial mass increases with speed but gravitational mass "remains the same." I have some choice comments about special relativity, but I don't want to spend all my waking hours at this keyboard, ok? <:-)

  • At cosmological scales the symmetry of the gravitational field does define a specific state of motion called the "Hubble flow". That doesn't mean Einstein was wrong. The world contains lots of specific things moving in specific ways, but the laws of physics don't.
  • The idea that objects get their inertia from their fields is legitimate, but I don't think it's relevant here.
-- BenRG (talk) 22:02, 10 December 2013 (UTC)[]
The idea that there is no "special" reference frame in physics is of course true. But from the perspective of useful definitions of "intertial mass" or "rest mass" to use an old-fashioned term - the reference frame of the object itself is a handy one to pick when defining these terms. In the frame of the object itself, inertial mass and gravitational mass are always the same - so it's convenient to just talk about "mass" - and for almost all applications of terms like "inertia", where velocities are small compared to c, we can greatly simplify the discussions by simply discussing "mass". That's important here because we need to make this article approachable to "ordinary" people for whom relativity is largely irrelevant and knowing why their big-assed SUV is going to crush my Mini Cooper like a bug is something they really should understand! SteveBaker (talk) 14:57, 11 December 2013 (UTC)[]

Thank you for clarifying this point. You state "In the frame of the object itself, inertial mass and gravitational mass are always the same." I take that to mean when the observer is riding with the mass in its inertial frame, the gravitational mass and inertial mass are indeed always the same. Now let's move the observer back to the "at rest" frame and let him observe the mass on the moving frame. What I have been saying (apparently with some difficulty) was the "at-rest" observer sees change in the inertial mass but not the gravitational mass (for this conversation). If we keep the mass at rest and accelerate the observer, the moving observer see no change in either the gravitational or inertial mass. If the accelerated observer did see a change, we would have the twins paradox all over again, and GR showed that wasn't possible. Are we Ok on that? NOW let's attack the sentence in the article that has been giving us fits. It said that the gravitational mass and inertial mass are always the same (or words to that effect), and I keep inserting the words "at low speed" and someone keeps editing it back out. The sentence might be better stated as follows: "For an observer moving along with a mass, the gravitational mass and inertial mass are always identical. For a stationary observer watching a moving mass, the gravitational mass and inertial mass are the same at low speed, but different at high speed. Does that make sense? — Preceding unsigned comment added by Zee99 (talk • contribs) 01:24, 12 December 2013 (UTC) []

I don't know Steve's or Ben's views, but I'm not happy with either of your assertions. An accelerated observer, once he is travelling at constant speed relative to the body being observed, sees exactly the same masses (both gravitational and inertial) as he would see if the body itself had been accelerated to the same speed. I agree that there will be a time difference after deceleration (hence the twins paradox), but this is General Relativity. I cannot see why, when observed from a frame moving at constant relativistic speed, there would be a difference between inertial mass (taking into account the apparent relativistic increase), and the gravitational mass (taking into account the effects of the extra energy which has gravitational effects just like invariant mass). Dbfirs 08:39, 12 December 2013 (UTC)[]

You say you're not happy with either of my statements. Let's take the first one, and please tell me what you don't like."For an observer moving along with a mass, the gravitational mass and inertial mass are always identical." It looks good to me. Zee99 (talk) 01:01, 13 December 2013 (UTC)Zee99[]

Sorry, I should have said two of the three. I don't know anyone who would doubt that invariant mass is the same in both its inertial effect and its gravitational effect, at least within the limits of current experimental accuracy. Dbfirs 08:34, 13 December 2013 (UTC)[]

How fast is inertia?[edit]

What is the speed of inertia? — Preceding unsigned comment added by 77.103.166.101 (talk) 19:48, 4 November 2013 (UTC)[]

Answer: Inertia is the resistance an inertial mass to be accelerated. Once a force is applied to the mass, the resistance due to the acceleration propagates forward with the speed c + s and to the rear of the mass with the speed c - s, where s is the speed of the mass with respect to a stationary observer, and c is the speed of light. — Preceding unsigned comment added by Zee99 (talk • contribs) 18:59, 7 December 2013 (UTC)[]

What is this "+ s" and "- s" phenomenon? I've never heard of it. See Special relativity. Dbfirs 09:55, 8 December 2013 (UTC)[]

Sorry, but I'm not talking about light. I'm talking about the propagation of the gravitational field potential. But even Einstein used the terms " + s" and "- s"." See Einstein's Miraculous Year, page 124. — Preceding unsigned comment added by Zee99 (talk • contribs) 23:36, 8 December 2013 (UTC)[]

Post Script: I suppose we could also talk about relativistic Doppler which also has such components. — Preceding unsigned comment added by Zee99 (talk • contribs) 23:41, 8 December 2013 (UTC)[]

So could I detect the gravitational potential from a distant spaceship, and use this effect to communicate at speed "c + s"? Dbfirs 08:45, 12 December 2013 (UTC)[]

I don't know. I'd make no such claim. However, as you probably know, only the speed of light is treated as being constant. You might look at Einstein's paper in Miraculous Year, p. 124 where he says he takes the constancy of the speed of light into consideration, yet still has terms such as "c + s". Zee99 (talk) 01:09, 13 December 2013 (UTC)Zee99[]

New definition[edit]

I've just reverted this definition:

Inertia is the tendency of an physical object to remain in the state of rest, motion and direction until that object is exposed to some external unbalanced force. (provided by user:Priyank Koul)

In the interests of open discussion, I'm not entirely happy with the previous definition that I reverted to. Can anyone improve on it? What definition do the best sources use? Dbfirs 06:36, 13 July 2014 (UTC)[]

GR established that inertia is the gravitational coupling between matter and spacetime. — Preceding unsigned comment added by 2804:14C:8780:A082:4838:A97C:1D8F:7A86 (talk) 18:04, 27 June 2016 (UTC)[]

JFB80 (talk) 05:19, 16 November 2019 (UTC)[]

Origins of Inertia[edit]

I think we need an expanded section describing how iniertia may arise, Mach-ian, from distant masses in the universe, or any other acceptable theories as to how it arises.

Spope3 (talk) 14:36, 20 November 2016 (UTC)[]

I second that. I was actually looking for a discussion that that when I came to this page. Bgovern (talk) 19:46, 15 November 2019 (UTC)[]

@Spope3: Apologies that it has taken 3 years for a response to your request. Isaac Newton is credited with being the first to observe the phenomenon we now call inertia, and to describe it in his First Law of Motion, one of science's physical laws. His First Law is sometimes called the Principle of Inertia. In asking for information about how inertia arises you are asking for something more fundamental than Newton's Laws of Motion. I am happy with the notion that we have nothing more fundamental than Newton's Laws of Motion, and that we aren't able to explain how inertia arises. Dolphin (t) 02:43, 16 November 2019 (UTC)[]
The law of inertia originated with Aristotle who justified it by saying that if a body is in motion there is no reason for it to stop and similarly if it is at rest. His argument was repeated by DesCartes and others. I can give references if anyone is interested.JFB80 (talk) 05:19, 16 November 2019 (UTC)[]
JFB80 My mistake - Newton was not the first to observe the phenomenon we now call inertia. Thank you for providing that correction. Dolphin (t) 12:07, 16 November 2019 (UTC)[]

JFB80, you are not right with respect to Aristotle. He, in fact, proposed that REST (at the natural place) is the only natural state. MOTION, for him, is always a process (rather than a state). FabioMSL (talk) 02:20, 26 July 2021 (UTC)[]

It was needed a Galileo (1638) to humanity realize that "if a body is in motion there is no reason for it to stop"!!! FabioMSL (talk) 02:23, 26 July 2021 (UTC)[]

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Aristotle[edit]

I have undone this addition by user JFB80 (talk · contribs), because in the cited source https://ebooks.adelaide.edu.au/a/aristotle/physics/book4.html Aristotle does not say that "in a void" an object set in motion will continue in that motion because there is no reason why it should stop. Furthermore, the conclusion that he may consequently be considered the originator of the law of inertia or "Newton's first law of motion", is not in the source. Comments welcome. DVdm (talk) 08:46, 18 November 2019 (UTC)[]

Newton's Laws of Motion are generally considered to have been a "breath of fresh air" because they cast aside the centuries-old teachings of Aristotle, many of which were clearly incorrect. I agree that Aristotle cannot be considered the originator of the principle of inertia. Dolphin (t) 11:53, 18 November 2019 (UTC)[]
@dVdm Yes it is in the cited reference you didn't have to be so hasty in deleting -you could have raised the point here first. See the 7th paragraph of section 8, book 4 which says
"Further, no one could say why a thing once set in motion should stop anywhere; for why should it stop here rather than here? So that a thing will either be at rest or must be moved ad infinitum, unless something more powerful get in its way."
@Dolphin Certainly Newton's Principia was a breath of fresh air but this article is looking for origins which were very much less clear.They evolved over about two thousand years until Newton. It is interesting that according to the book of Herival (Background to Newton's philosophy. Oxford 1965), Newton had himself studied the statements of Aristotle on motion and in particular the passage quoted here. He had also given attention to a similar statement for bodies at rest in de Caelo.(On the Heavens) book iii chap 2 301b JFB80 (talk) 20:44, 18 November 2019 (UTC)[]
@JFB80: I concede that, long before Newton, others had observed and recorded the phenomenon we now call inertia. It is overstating the case to say Aristotle, or anyone else, "may consequently be considered the originator of the law of inertia or Newton's first law of motion." It may be accurate to say Aristotle's ideas were known to Newton and used by him in preparing his Laws of Motion. Dolphin (t) 12:28, 19 November 2019 (UTC)[]
@Dolphin5.1 Thank you for your reply. I still think there is a close connection with Aristotle because even in the early 1600's thinking was still very much under Aristotle's influence (remember Galileo 1620). The quoted reference from Stanford University on the history of Newton's laws remarks that Newton probably first encountered the law of inertia in print when he read Descartes' Principia Philosophiae 1644 known to be scholastic (i.e Aristotelian). The statement of the law of inertia given there is recognizably in Aristotle's style. Maybe it was after reading this statement Newton himself studied Aristotle as remarked above.JFB80 (talk) 15:56, 20 November 2019 (UTC)[]
@JFB80: Thanks for that clarification. I will accept that Aristotle can be considered the originator of the principle of inertia. I will modify my earlier comment to say it is overstating the case to say "Aristotle was the originator of Newton's first law of motion." I say this primarily because Aristotle pre-dated Newton by many centuries. Dolphin (t) 04:43, 21 November 2019 (UTC)[]
On the other hand, note that if indeed Newton probably first encountered the law of inertia in print when he read Descartes' Principia Philosophiae 1644 known to be scholastic, that is still wholly insufficient to write that Aristotle may be considered the originator of the law of inertia or "Newton's first law of motion" without a proper source that directly backs it. Doing so would still be a schoolbook example of wp:original research by wp:synthesis. Stanford talks about Newton and Descartes and mentions scholastics. We certainly cannot conclude anything about Aristotle from that. - DVdm (talk) 10:06, 21 November 2019 (UTC)[]
The important thing is not that Aristotle the man lived a very long time ago and pre-dated Newton by many centuries but that his teachings had been very influential up to the time of Newton and later in France under the influence of DesCartes until Voltaire introduced Newtonian ideas. JFB80 (talk) 14:58, 21 November 2019 (UTC)[]
Sure, that could be important, but without very reliable sources explicitly stating all this, we cannot include it in our articles. - DVdm (talk) 16:16, 21 November 2019 (UTC)[]
I do not intend to put all these ideas into the article. That would be a big job. Originally all I wanted to do was to point out that the usual accounts of what Aristotle said on motion are incomplete because they omit that he said that in a void, a body in motion will continue in the same motion i.e. what is now called the principle of inertia. JFB80 (talk) 06:36, 22 November 2019 (UTC)[]
@DVdm Would you have any objection if I re-introduced my previous comment into the article without the conclusion that Aristotle was the originator of the principle of inertia? I have shown exactly where it was written in Physica JFB80 (talk) 18:56, 23 November 2019 (UTC)[]
The source for motions in a void has been added to the article. JFB80 (talk) 05:59, 24 November 2019 (UTC)[]
No problem with this addition. - DVdm (talk) 09:08, 25 November 2019 (UTC)[]
Retrieved from "https://en.wikipedia.org/w/index.php?title=Talk:Inertia&oldid=1035529775"

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