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Inertia: it's not just for sitting around any more.
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2014/05/06
2:50pm
Ron Draney
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Was a little disappointed when at the end of the discussion on the everyday meaning of inertia ("things tend to remain at rest") vs the technical meaning ("things tend to remain at rest or in motion"), nobody offered the poor guy a substitute term without the ambiguity. The caller's son needs to develop, not inertia, but momentum.

2014/05/06
3:15pm
New River, AZ, USA
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OK, I just listened to that podcast. Totally agree with Ron. Momentum is a far better word to describe what the guy was asking about, and whether or not Grant knows that momentum = mv (and I suspect he does), it's a word that has entered common use in sports, politics, economics, etc. It was scientifically defined in Newton's Principia ca. late 17th century. But take a look at this interesting result from Ngrams.

The rapid drop after 1800 is likely due to the fact that, by then, there wasn't much more to say about physical momentum. The gradual rise after that is certainly due to the term's incorporation as a metaphor for continued progress.

Note that my Ngram search was for "their momentum" in order to bias the results toward metaphorical uses. Physicists don't often speak of momentum collectively.

2014/05/06
7:50pm
Dick
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I may be wrong about this but the way I understand it, inertia of a specific mass never changes.  It always has the same resistance whether it is resistance to move or resistance to stand still.  Momentum changes.  I have not heard the podcast with this but from the previous comments it sounds like the son needs momentum.  That's what so many sons need.

2014/05/07
12:15am
deaconB
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Inertia is a function of mass only.  Momentum, on the other hand, is a function of both mass and vector. When pool balls collide, their inertia does not change, but their momentum certainly does.

Within the confines of Newtonian physics, of course. 

There is no ambiguity.  Both the gas pedal and the brake pedal work to defeat inertia.

2014/05/07
12:54am
Robert
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Momentum is what you already have:  politicos , athletes ride on the momentum from the last victory.

To develop momentum, you first need energy from external source: a victory, getting hit by another pool ball, a kick in the pants.

Inertia in the real world actually matches the technical sense just fine:   You sit around not wanting to do anything, or do the same job day in day out wishing no advancement.   (In the first case you have no momentum, in the 2nd you  have some momentum handy in case a  better job comes along.)

2014/05/07
10:32am
deaconB
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Robert said
Momentum is what you already have:  politicos , athletes ride on the momentum from the last victory.

To develop momentum, you first need energy from external source: a victory, getting hit by another pool ball, a kick in the pants.

Not necessarily external.    Remembering the engine that said I think I can, I think I can works as well as an externally applied kick in the pants.  And firing up that JATO unit on the roof of your Chevelle works, too, at least until you and your Chevelle paint the scenery.  Which reminds us that brakes, including a drag chute, develop negative momentum,  ad that's internal as well.

Inertia in the real world actually matches the technical sense just fine:   You sit around not wanting to do anything, or do the same job day in day out wishing no advancement. 

And praying that he won't get RIFfed.

A so-called "professional" baseball player can wish all he wants, but being a slave, he really cannot advance himself. That can only happen as an acceleration applied by his master, who decides he should pitch in New York rather than Columbus.

2014/05/07
3:12pm
New River, AZ, USA
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Just to be perfectly clear, inertia is a function of both mass and speed. Relativistic effects increase the inertia of an object in motion, but as deaconB points out with his "Newtonian physics" comment, for most of the speeds we are likely to encounter, it's essentially a function of mass.

Interestingly, the protons circulating in the Large Hadron Collider have their inertia/mass increased by a factor of 7500. But then, they are moving at 99.9999991% the speed of light.

One more point … for something to change its momentum, there must be an interaction with something external. An attached JATO or drag shoot might seem like "internal" causes, but both interact with something external to the system. In the case of the drag chute, it's the ambient atmosphere. In the case of the JATO, it's the exhaust being expelled. The Little Engine That Could was interacting via friction with the rails under its wheels.

I still think the metaphor applies well in non-scientific circles (politics, sports, etc.), but as with all metaphors, it can be stretched too far.

2014/05/07
11:00pm
Robert
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Mass times velocity is momentum.  Here.

Inertia is not a calculated quantity. It is the concept that an object maintains constant velocity (or stays stationary) if not acted upon. See here.

Well there might be interchanged usages between the 2, but that's not strictly correct .   

If so, this is one case where lay language does a better job to adhere with the strict technical terminology:  An inert person is one who keeps doing the same thing or keeps doing nothing (hence resisting change);  and a politico with momentum is one moving fast with great 'mass' of supports and money.

2014/05/08
1:32pm
deaconB
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Heimhenge said
Just to be perfectly clear, inertia is a function of both mass and speed. Relativistic effects increase the inertia of an object in motion, but as deaconB points out with his "Newtonian physics" comment, for most of the speeds we are likely to encounter, it's essentially a function of mass. 

The mass of an object increases as speed increases.  Isn't inertia still equal to mass as that happens?

If you had an object in space, and you removed the inertia, the sunlight falling on it would immediately accelerate it to light speed, I think.  Not the key to traveling backwards in time, but it would stop time.  Moving forward in time is less problematic;  I've moved forward from the Truman administration to Obama with only the most primitive technology: a piece of paper upon which was inscribed off symbols declaring it to be a birth certificate.

A scientific term that might be more appropriate to not advancing in career, etc., might be reluctance, which must be overcome to magnetize something.  I always had to fight reluctance, whether I was waking up or falling asleep, I was going to work or I was leaving it. 

2014/05/08
6:49pm
New River, AZ, USA
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You can't really say "inertia is equal  to mass" since they measure different properties. Inertia is resistance to acceleration, measured in units of "kg" as defined by Newton's 2nd Law: m = F/a. Mass, on the other hand, is measured as an amount of matter defined by the "standard kilogram." Physicists think that they're both the same thing, but there's no real consensus. It gets into the thorny equivalence principle.

But yes, however you interpret the equivalence principle, inertia is proportional  to mass (with a proportionality constant of 1.0 as far as can be determined), and both will increase with speed in accord with special relativity.

deaconB said: A scientific term that might be more appropriate to not advancing in career, etc., might be reluctance, which must be overcome to magnetize something.

Nice.

2014/05/08
7:35pm
CheddarMelt
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When I'm in bed, I don't want to get up.

When I'm up, I resist going to bed.

I've long referred to that as sleep inertia.

2014/05/08
11:02pm
deaconB
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Heimhenge said
You can't really say "inertia is equal  to mass" since they measure different properties. Inertia is resistance to acceleration, measured in units of "kg" as defined by Newton's 2nd Law: m = F/a. Mass, on the other hand, is measured as an amount of matter defined by the "standard kilogram." Physicists think that they're both the same thing, but there's no real consensus. It gets into the thorny equivalence principle.

It's been a long time since college physics. They taught us, for instance, that there were four states of matter, not five.  Just as a matter of curiosity, how do dissenters define mass other than as resistance to a change in vector?

2014/05/09
12:36am
New River, AZ, USA
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deaconB said: Just as a matter of curiosity, how do dissenters define mass other than as resistance to a change in vector?

That's the thorny question I referred to. It's not so much a question of mass/inertia resisting a change in momentum. We understand that. It's the definition of "mass" itself that underlies the problem.

This isn't a "physics forum," but since you asked … the "m" in m=F/a (the directly perceived mass based on acceleration in response to a force) and the "m" in Newton's gravitational equation m=aR^2/G (the mass that responds to gravity) are assumed to be equal. But there's no physical theory that can prove that at present.

Dissenters would argue that the inertial mass and the gravitational mass are not necessarily equal. And there is no theory that "proves" they are the same. But attempts to measure any difference have to date shown no difference. Still, there remains the question: Is the mass that gives rise to inertia the same as the mass that gives rise to gravity? Physics at present has no answer.

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