Talk:Inertia

Inertial frame

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)

"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)

inertia is a force

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)

Caption

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

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

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

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?

"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

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

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 so that they can swing like a pendulum. 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 a mass 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)