# The Concept Of Mass

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### #137 VictorMedvil

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Posted 14 April 2019 - 05:42 AM

Doesn't accelerate?   Drive a car into into a concrete wall sometime, eh, Vic?  The inertia of that wall will accelerate the hell out of the car, even if it does nothing more than stop it dead in its tracks.  That is acceleration.  And it is the car, not the wall, which gets accelerated.

I don't see any significant difference between "moving" and "resisting movement" in this case..  A force doesn't have to consist of kinetic energy.

You are talking about the normal force not friction or interia, that is something different that is the object's motion being redirected by a solid object. It doesn't accelerate the object it actually slows and moves in another direction even though you may barely notice it. If the object was being accelerated it would move faster rather the crash.

Car crashing into a wall

Edited by VictorMedvil, 14 April 2019 - 05:46 AM.

### #138 OceanBreeze

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Posted 14 April 2019 - 05:45 AM

Actually, Newton himself described inertia as a force:

https://en.wikipedia.org/wiki/Inertia

In standard Newtonian mechanics, inertia is not treated as a force and it is not shown as a vector on free body diagrams.

Newton’s second law, F = ma, says that a body with mass m, and an unbalanced force F applied to it, will undergo an acceleration a, describing a dynamic situation.

According to D'Alembert's principle, the dynamic situation is changed to a static one by simply considering the RH side as a negative inertial force such that F - ma = 0.

In doing this, the body can be considered to be in static equilibrium under the actions of the real force F and the fictitious, inertial force -ma.

To be honest, this is something that I do automatically when working mechanics problems and have never really considered it to be a separate principle from Newton’s second law, but formalists must be formalists!

Admittedly, however, it is easier to work a problem in statics than in dynamics, so the principle does have merit and probably deserves its own name.

### #139 Moronium

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Posted 14 April 2019 - 06:01 AM

To be honest, this is something that I do automatically when working mechanics problems and have never really considered it to be a separate principle from Newton’s second law, but formalists must be formalists!

Admittedly, however, it is easier to work a problem in statics than in dynamics, so the principle does have merit and probably deserves its own name.

To date, nobody has shown that D'Alembert's principle is equivalent to Newton's Second Law...However, an approximate solution to this problem does exist.

GR claims that gravity is "not a force."  A lot of this just seems to be semantics.  Treating gravity as a force gets us to the moon and back.  Such "approximation" is good enough for government work, eh?

### #140 Moronium

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Posted 14 April 2019 - 06:11 AM

You are talking about the normal force not friction or interia, that is something different that is the object's motion being redirected by a solid object. It doesn't accelerate the object it actually slows and moves in another direction even though you may barely notice it. If the object was being accelerated it would move faster rather the crash.

Still not following your reasoning here, Vic.  You say: "If the object was being accelerated it would move faster rather the crash." It seems to me that you are attempting to limit "acceleration" to "increased speed."  But acceleration is ANY change in speed or direction.  That would include stopping, right?

And it takes a "force" (like friction, for example) to slow a moving object, right?

Edited by Moronium, 14 April 2019 - 06:15 AM.

### #141 ralfcis

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Posted 14 April 2019 - 06:41 AM

So resistance is also now a force? Are circuit boards also now forces. Are wires also forces. If I took a wire and shorted out a capacitor in a circuit, I'm introducing something outside the circuit's normal behavior just like you applying brakes to a car. The mass of the car does store and return force just like a capacitor in a circuit but you're equating the storage device with the stuff it stores. An oil drum is not oil, mass is not the force it stores and since mass=inertia, inertia is also not a force.

Edited by ralfcis, 14 April 2019 - 07:09 AM.

### #142 VictorMedvil

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Posted 14 April 2019 - 07:06 AM

Still not following your reasoning here, Vic.  You say: "If the object was being accelerated it would move faster rather the crash." It seems to me that you are attempting to limit "acceleration" to "increased speed."  But acceleration is ANY change in speed or direction.  That would include stopping, right?

And it takes a "force" (like friction, for example) to slow a moving object, right?

Its not a force in the form that you are describing it doesn't make things gain motion, it only slows the motion of objects. It would be like saying a wire is a force because it has electrical resistance which is not true but Ralfcis has the proper idea.

Edited by VictorMedvil, 14 April 2019 - 07:11 AM.

### #143 Moronium

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Posted 14 April 2019 - 07:15 AM

Its not a force in the form that you are describing it doesn't make things gain motion, it only slows the motion of objects. It would be like saying a wire is a force because it has electrical resistance which is not true but Ralfcis has the proper idea.

You didn't respond to the questions I asked at all, Vic.  You just re-asserted your mistaken notion ("Its not a force in the form that you are describing it doesn't make things gain motion").

I hate to have to say it, but you're starting to sound exactly like Awol.

Edited by Moronium, 14 April 2019 - 07:20 AM.

### #144 ralfcis

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Posted 14 April 2019 - 07:21 AM

I think Awal gave up his attempts at trying to impress the birds at the pub with his science knowledge because he hasn't been heard from in a while.

Edited by ralfcis, 14 April 2019 - 07:32 AM.

### #145 Moronium

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Posted 14 April 2019 - 07:26 AM

you're equating the storage device with the stuff it stores. An oil drum is not oil, mass is not the force it stores and since mass=inertia, inertia is also not a force.

Another irrelevant and misconceived analogy of yours, eh, Ralf?  What "storage device" are you even talking about?

### #146 Moronium

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Posted 14 April 2019 - 07:28 AM

I think Awal gave up his attempts at trying to impress the girls at the pub with his science knowledge because he hasn't been heard from in a while.

Other way around, Ralf.  Awol  hasn't showed his face around here precisely because he discovered that he won't get any positive reception to his occult-like pseudo-scientific metaphysical pronouncements unless he is dealing with chumps.  He keeps to the bars now.

Edited by Moronium, 14 April 2019 - 02:20 PM.

### #147 ralfcis

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Posted 14 April 2019 - 07:36 AM

"What "storage device" are you even talking about?"

Try to follow along. I've compared mass in  a mechanical circuit to a capacitor in an electrical circuit. Both are storage devices of force.

### #148 Moronium

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Posted 14 April 2019 - 07:43 AM

"What "storage device" are you even talking about?"

Try to follow along. I've compared mass in  a mechanical circuit to a capacitor in an electrical circuit. Both are storage devices of force.

Like Awol, you just re-assert your conclusion.  Where's the "storage device" with inertia?  What is it?  You don't put an oil barrel into your oil-burning furnace.  You put in the oil, that's all.  You're the one who can't see the difference.

Edited by Moronium, 14 April 2019 - 07:47 AM.

### #149 Moronium

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Posted 14 April 2019 - 07:56 AM

By the way, Ralf, when you use the word "pubs," you reveal something I should have realized a long time ago.  You're a damn Limey, aint ya?  That explains a lot, right there.

### #150 Moronium

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Posted 14 April 2019 - 08:07 AM

Without what I'm now calling "the force of inertia," you couldn't even throw a baseball.  It would just drop straight to the ground the second it left your hand.

This is the same error the ancients made (and which Galileo corrected) when they concluded that it was "impossible" for the earth to be moving.

Their argument was that if you threw a ball straight up into the air, it could not come "straight down" to you (as it does) if the earth were moving, because in that case you would have moved "out from under it" while it was in the air.

Edited by Moronium, 14 April 2019 - 08:14 AM.

### #151 ralfcis

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Posted 14 April 2019 - 08:13 AM

I was born in England a damned Polack but lived here in racist Canadia 60 yrs.

### #152 Moronium

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Posted 14 April 2019 - 09:21 AM

Newton’s second law, F = ma, says that a body with mass m, and an unbalanced force F applied to it, will undergo an acceleration a, describing a dynamic situation.

Well, yeah, Popeye, but that is quite consistent with the point, as I see it.

What is an "unbalanced force?"  It is essentially a "net force," right?

Many different forces might need to be taken into consideration before you can arrive at a net (unbalanced) force.  Like the force of friction, for example.  Or "the force of inertia" for another, eh?

Even if there are no "unbalanced" forces in play, and therefore no acceleration, there can still be a multiplicity of forces which are "acting upon" an object.

Looking at it that way, you could say that the "force of inertia" is the force that keeps a moving object moving.

Edited by Moronium, 14 April 2019 - 09:46 AM.

### #153 Moronium

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Posted 14 April 2019 - 09:34 AM

I have seen the argument made that inertia cannot be a force because an object "at rest" does not start moving.

But consider a large boulder on the ground, not moving at all.  But for the the "inertia force" supplied by the "mass" of the earth, the boulder would head straight to the earth's center at an ever-accelerating speed.  It would just go through the earth's crust like it was air.

Another way to look at it is that the "inertia" of the boulder is opposing the force of gravity, including the gravity of the moon, the sun, and all other matter.  Without that force, it would move.  The boulder's inertial force has simply reached an equilibrium with all other relevant forces, and hence it remains motionless.  There is no "unbalanced" force in play and therefore its acceleration is zero.

The tides don't happen because surface water has no "force of inertia" to resist the moon's gravitational pull.  Water just doesn't have "enough" opposing force to completely resist and offset it, like a boulder does.

Same idea with the moon orbiting the earth, where an equilibrium between opposing forces has been reached. As I have noted before, at least two forces must be in play for that to happen:

1. The force of earth's gravity, which is "tugging at" the moon, and

2.  The opposing force of inertia, which resists, and offsets, this tug. The force of Inertia "tries" (and keeps trying, to all eternity) to keep the moon moving in a straight line, but it is not completely successful in doing so.  All the same, without it, the moon would just drop straight into the earth.

Edited by Moronium, 14 April 2019 - 10:50 AM.