Wikipedia:Reference desk/Archives/Science/2019 January 18

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January 18

Inertia vs mass

What is the difference exactly between inertia and mass in the units in which they are expressed?. If the units are the same, are the quantities the same? 86.8.202.148 (talk) 00:40, 18 January 2019 (UTC)

Same dimensions, same units (kilograms), and same values. Yes, their quantities are the same.

They'd be the same for different gravitational fields too. That's not the case for weight, which varies according to the local gravity.

Inertia isn't the same thing as mass, but it is one of the physical manifestations of it, as inertial mass: the resistance of objects to an applied froce causing them to accelerate, per Newton's 2nd law. It's different as a concept from gravitational mass, see Mass#Inertial vs. gravitational mass, but the two have always been measured as equal, and (post-Newton) this has been given a theoretical basis as the equivalence principle. Andy Dingley (talk) 01:38, 18 January 2019 (UTC)

Ok. How can we price that inertia and mass are different things? — Preceding unsigned comment added by 86.8.202.148 (talk) 02:39, 18 January 2019 (UTC)

"Price"? ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:58, 18 January 2019 (UTC)

I'm not quite sure what you mean. As Andy says, inertial mass is conceptually different from gravitational mass. The equality of inertial mass and gravitational mass is a synthetic rather than analytic proposition, one empirically observed to be true and for which we have good reason to think it should always be true, but it is not logically impossible that they could be different.

As far as contrasting "inertia" with mass — Andy seems to be taking inertia to mean inertial mass, but (as a linguistic rather than scientific point) I don't think that's correct. My understanding of the word "inertia" is that it describes a principle or a concept, not a quantity. You can't measure the inertia of something any more than you can measure the electromagnetism of something. So inertia doesn't have units. Again, this is a point about the meaning of the word, not about the deeper concepts. --Trovatore (talk) 04:13, 18 January 2019 (UTC)

• Inertia and mass are different because we defined them to be different. They are just words after all, and we choose to apply them to concepts of our choice. There are three aspects to 'mass' – three fundamental physical behaviours that we observe. One of these we just call 'mass'. It's the behaviour that as atoms have mass, we can (with highschool physics and chemistry) sum that up to find molecular masses, then (with Avogadro's constant) bulk masses too. Tracking that further back to sub-atomic particles, we know that protons, neutrons etc. have mass, and that (with some atomic physics and the concept of binding energy) gives us those atomic masses. But going deeper than that is quite recent and into Higgs boson territory.

The other two aspects are empirical observations: we take a 'massive' particle and find that it has two distinct behaviours as a result. One is gravitational mass, i.e. the thing which gives rise to weight. Then it also has inertial mass, its resistance to acceleration. Recognising that those two are distinct, but consistently always the same value, is itself quite a deep insight, via the equivalence principle.

So really, your question is about "How are these three things related", not just two. Andy Dingley (talk) 10:34, 18 January 2019 (UTC)

Not sure what you mean by "my" question. Yes, there are three things involved, that's true. But none of the three is called simply "inertia". Inertia is not a quantity and does not have units. As I say, it's not a deep point, but it is a point. --Trovatore (talk) 22:18, 18 January 2019 (UTC)

Per Andy, who gave some excellent definitions, there's also something here to remember with regard to terminology. Inertia is a property of something's motion, specifically it is the effect of mass on momentum. The real question is why we have multiple different masses. Each kind of mass we measure in a different way. We have inertial mass which is mass we get by measuring the effect of that mass on changes in velocity, but we also have gravitational mass, which is we get by measuring the force of attraction between two bodies. Inertial mass comes from Newton's second law, whereas gravitational mass comes from Newton's law of gravitation. We also have rest mass and relativistic mass, which are the masses we measure when we are in the same or co-moving frame of reference as a body (rest mass) and when we are in a different frame of reference (relativistic mass). The real question is why measuring the effect of inertia on an object should be identical to measuring the effect of gravity on an object. Before Einstein, we only knew that careful measurements of both gave identical values, but we had no good theory to explain why those measurements were always the same. There was nothing at the time to predict that they should be the same; which is why we have two definitions. General relativity gave us the theoretical framework to explain why they had to be the same value. --Jayron32 14:38, 18 January 2019 (UTC)

ok why not article say inertia is an inherent property of mass cos of Newtowns law. But does not exist without mass. In fact it's fictitious, like suction. Just another way of staring Isaacs laws. 80.2.22.165 (talk) 23:32, 19 January 2019 (UTC)

I agree with Trovatore. Mass is a measurable quantity with units such as the kilogram. Inertia is a property that is possessed by all things with mass. A mosquito and an elephant both display the property of inertia but it isn’t correct to say the elephant has more inertia than the mosquito. The elephant has a much greater mass than the mosquito, and they both display inertia.

Newton’s first law of motion is often called the principle of inertia. It says if the resultant force acting on an object is zero, the momentum of the object remains constant. If the resultant force on an elephant is zero, the elephant’s momentum remains constant, but it isn’t any more or less constant than that of a mosquito which is experiencing a zero resultant force. Dolphin (t) 23:42, 21 January 2019 (UTC)

Do magnets attract flying sparks?

Does anyone have the answer or can they point me towards any videos or papers on this? I did a search expecting tons of cool videos showing sparks off a grinder being bent upward or attracted to a magnet, but drew blanks. I know heat can ruin the magnetism of iron, but can a strong enough magnet counteract that? ^Thanks,L3X1 ◊distænt write◊ 02:33, 18 January 2019 (UTC)

No. Iron/steel that is glowing red is above the curie temperature and thus stops being ferromagnetic. It is paramagnetic, but the forces involved are far weaker. --Guy Macon (talk) 04:57, 18 January 2019 (UTC)

• Yes, magnets will do. This is familiar to any welder using magnetic clamps, and having to wipe dusty, ferrous crud off them.

The point about Curie point is a good one but the Curie temperature for iron is fairly high (it's typically a red heat). As sparks are small, they cool very quickly and will be below their Curie point within inches. If you have sparks that leave orange-yellow glowing trails, that's because they were hot enough to ignite and so they're not just hot as they fly, but they're actually still burning and maybe getting hotter (glowing sparks this hot aren't ferromagnetic, so aren't attracted to magnets). Some sparks even burst into bright fireworks, sometimes when they hit another surface. This is because they're hot, weakened by the heat and break up on impact or after time. This break up increases their surface area and so they burn suddenly brighter before going out. You can (to some extent) tell the alloy of unknown steel by spark testing it and looking at the shape and colour of these sparks. Some of these sparks (cast iron, wrought iron, some stainless) can indeed be magnetic as soon as they're formed. Andy Dingley (talk) 10:44, 18 January 2019 (UTC)

For reference, dark red heat is from 704°C to 814°C[1] and the Curie point of iron is 770°C.[2] So the OP wouldn't see the bright sparks from the grinder being attracted to the magnet, but the magnet would end up covered with the less-visible cooler bits of iron or steel. --Guy Macon (talk) 17:36, 18 January 2019 (UTC)

I had noticed that once and figured I was just not quick enough to spot anything, now I know I'd be unable to see those tiny pieces period as they were never sparks. Thanks all, ^Thanks,L3X1 ◊distænt write◊ 22:45, 18 January 2019 (UTC)