Physics Help Forum Kinetic energy relation to mass of an object.

 Special and General Relativity Special and General Relativity Physics Help Forum

Jul 8th 2017, 07:59 PM   #11
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 Originally Posted by kiwiheretic Yeah, I was a little confused about the differences between inertial mass, gravitational mass and relativistic mass after watching this video: However it does make more sense if they are the same.
Have just watched that video ... the guy certainly does come over as an expert on this subject ...nonetheless I have some problems with what he says ....

@ 4:30 he says gravitational mass and inertial mass are completely different!!! ...he says a "it turns out from newton's point of view it was completely an accident ...hard to understand why that's true " (that gravitational and inertial mass are the same) ...

I don't understand this at all .... Yes newton defined mass as F=ma ...then by setting Big G in the formulae at 6.67 this made gravitational and inertial mass quantitatively the same ... but they always were Qualitatively the same... By this I mean if an object has an increased mass because of velocity , this will cause it's gravitational pull on nearby masses to be greater ..... there's only one mass!

Jul 8th 2017, 09:36 PM   #12

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 Originally Posted by oz93666 Have just watched that video ... the guy certainly does come over as an expert on this subject ...nonetheless I have some problems with what he says .... @ 4:30 he says gravitational mass and inertial mass are completely different!!! ...he says a "it turns out from newton's point of view it was completely an accident ...hard to understand why that's true " (that gravitational and inertial mass are the same) ... I don't understand this at all .... Yes newton defined mass as F=ma ...then by setting Big G in the formulae at 6.67 this made gravitational and inertial mass quantitatively the same ... but they always were Qualitatively the same... By this I mean if an object has an increased mass because of velocity , this will cause it's gravitational pull on nearby masses to be greater ..... there's only one mass!
The two masses do seem to make the case for equality but there is no reason, physically or philosophically, that requires inertial mass and gravitational mass to be the same thing. Admittedly the ratio between the two is very close to 1 but that doesn't constitute a proof.

-Dan
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Jul 8th 2017, 10:07 PM   #13
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 Originally Posted by topsquark The two masses do seem to make the case for equality but there is no reason, physically or philosophically, that requires inertial mass and gravitational mass to be the same thing. Admittedly the ratio between the two is very close to 1 but that doesn't constitute a proof. -Dan
Very close!! has any experiment show any difference at all?

"No reason physical or philosophically reason for the two to be the same? !!" Physics is about experiment which shows them the same ... so this should be a big clue to figuring out what exactly mass is , cos at the moment no one knows ...

 Jul 9th 2017, 03:08 AM #14 Senior Member     Join Date: Feb 2017 Posts: 205 Proof that inertial mass and gravitational mass is one and the same thing. F= ma, Also F = GMm / r². so if inertial mass and gravitational mass are the same then ma= GMm / r² which gives us a= GM / r². Now we calculate a. (The m in ma is the inertial mass and the m in GMm / r² is gravitational mass) a= GM / r² = (6.67 × 10^-11)(6.0 x 10^24) / (6.4 x 106)² = 9.77ms-2. Now we have observed that 2 objects thrown from the same height reach the ground at the same time. We all know that acceleration due to gravity is 9.8 m/s². If our observation is right then gravitational mass should be same as inertial mass. Last edited by avito009; Jul 9th 2017 at 03:32 AM.
Jul 9th 2017, 08:53 AM   #15
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 Originally Posted by oz93666 Have just watched that video ... the guy certainly does come over as an expert on this subject ...nonetheless I have some problems with what he says .... @ 4:30 he says gravitational mass and inertial mass are completely different!!! ...he says a "it turns out from newton's point of view it was completely an accident ...hard to understand why that's true " (that gravitational and inertial mass are the same) ... I don't understand this at all .... Yes newton defined mass as F=ma ...then by setting Big G in the formulae at 6.67 this made gravitational and inertial mass quantitatively the same ... but they always were Qualitatively the same... By this I mean if an object has an increased mass because of velocity , this will cause it's gravitational pull on nearby masses to be greater ..... there's only one mass!
And other more prominent physicists say the opposite like Hans C. Ohanian

Jul 9th 2017, 04:18 PM   #16
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 Originally Posted by Pmb And other more prominent physicists say the opposite like Hans C. Ohanian
When I was taught physics many decades ago , there was no talk of different kinds of mass ... I was taught there is ONE mass , measured in Kg ...and it is the property which resists acceleration , and causes gravity ....

If Mr Ohanian is suggesting there are now two kinds of mass he needs to offer experiments which can prove this hypothesis showing gravitational and inertial mass are not the same ...otherwise it's just talk ... marks on blackboard ... unproven theory.

Jul 9th 2017, 07:57 PM   #17
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 Originally Posted by oz93666 When I was taught physics many decades ago , there was no talk of different kinds of mass ... I was taught there is ONE mass , measured in Kg ...and it is the property which resists acceleration , and causes gravity .... If Mr Ohanian is suggesting there are now two kinds of mass he needs to offer experiments which can prove this hypothesis showing gravitational and inertial mass are not the same ...otherwise it's just talk ... marks on blackboard ... unproven theory.
Well, one experiment might be to fire an object to a known velocity at a spring oriented horizontally and then finding the maximum compression of the spring and using Hooke's law, calculate the inertial mass, then conduct the same experiment place the object on a spring scale and then again measure the gravitational mass and then check if these two quantities are equal.

Not sure if its possible to measure relativistic mass in this way

 Jul 16th 2017, 02:10 PM #18 Physics Team   Join Date: Apr 2009 Location: Boston's North Shore Posts: 1,576 I'd also like to point out that when many physicists, mostly particle physicists, use the term "mass" they're referring to the particle's proper mass (aka rest mass). Some people just love to make a big stink about this but the entire squabble is all about how and why one defines the term mass. I was actually tossed out of a forum or two because I use the term as its used in this thread and when someone claimed that mass doesn't depend on speed I explained that there is a debate about it. You would not believe how po'd some people will get when you do that. Wow! Lol!!
Jul 16th 2017, 03:54 PM   #19
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 Originally Posted by oz93666 When I was taught physics many decades ago , there was no talk of different kinds of mass ... I was taught there is ONE mass , measured in Kg ...and it is the property which resists acceleration , and causes gravity ....
Whoever taught you that was wrong.

 Originally Posted by oz93666 If Mr Ohanian is suggesting there are now two kinds of mass he needs to offer experiments which can prove this hypothesis showing gravitational and inertial mass are not the same ...otherwise it's just talk ... marks on blackboard ... unproven theory.
In the first place nobody ever said that it was Ohanian's theory. I said he is an example of one who defines it in this texts. But it'd be difficult to find a decent text which doesn't define gravitational mass. When I started college in the early 80's the text everyone was using for basic physics was

The Fundamentals of Physics by Halliday and Resnick

Nowadays one uses a text like

Physics For Scientists and Engineers by Knight

If you search online you might be able to find a place to download a copy of them to read for yourself. Try this to start

https://archive.org/details/Fundamen...yResnickWalker

Each of these texts defines the term "gravitational mass." In any case its all been tested experimentally to high degrees of accuracy.

I wrote an entire paper on the concept of relativistic mass which includes all three kinds of mass. See: https://arxiv.org/abs/0709.0687

In this paper I describe these concepts and their origins. But the definitive books on the subject were written by Max Jammer. They are

Concepts of Mass in Classical and Modern Physics

Concepts of Mass in Classical and Modern Physicsby Max Jammer | Review by: C. Truesdell | download

Concepts of Mass in Contemporary Physics and Philosophy

Concepts of mass in contemporary physics and philosophy: Max Jammer; Princeton University Press, Princeton, 2000, xi+180pp., price US \$41.50, ISBN 0-691-01017-X | Roberto Torretti | download

Each of these define and explain the three concepts of mass as I do in my own paper. They are

inertial mass – Mass which resists changes in momentum
passive gravitational mass – Mass acted upon by gravity
active gravitational mass – Mass that is the source of gravity

You'll find these concepts defined almost everywhere in physics. For example, see

https://en.wikipedia.org/wiki/Mass
 Inertial mass measures an object's resistance to being accelerated by a force (represented by the relationship F = ma). Active gravitational mass measures the gravitational force exerted by an object. Passive gravitational mass measures the gravitational force exerted on an object in a known gravitational field.
The important thing to understand here are the different physical concepts of mass. One can choose the gravitational constant in such a way that they all have the same numerical value.

 Originally Posted by oz93666 If Mr Ohanian is suggesting there are now two kinds of mass he needs to offer experiments which can prove this hypothesis showing gravitational and inertial mass are not the same ...otherwise it's just talk ... marks on blackboard ... unproven theory.
You're thinking like a mathematician now, not like a scientist. First off physics is not about proving anything. If someone every told you that something was proven in physics then they were wrong. Do yourself a favor and watch this video: http://www.newenglandphysics.org/com...an_Guth_04.mp4

As for marks on a blackboard - Well my friend, you'll just have to learn about these concepts before we go past here. Until you learn about it you won't understand what's wrong with that statement. Read Max Jammers books and you'll understand the physics. I'll give you a preview

Newton's Law of Motion: F = dp/dt where p = m[sub]i[/sub]v

where m[sub]i[/sub] is defined as the inertial mass of the particle

Newton's Law of Gravitation: F = HMm/r[sup]2[/sup]

where H is a constant of proportionality

M is the source of gravity

M acts on m

M is defined as the active gravitational mass

m is defined as the passive gravitational mass

Einstein's Principle of Equivlenence states

m is proportional to m[sub]i[/sub], i.e. m = k*m[sub]i[/sub]

Therefore

F = HMm/r[sup]2[/sup] = HM(k*m[sub]i[/sub] )/r[sup]2[/sup]

F = GMm[sub]i[/sub] )/r[sup]2[/sup]

G is defined as the gravitational constant

This is all standard stuff defined in all college level basic physics textbooks. Have you ever read one? Most don't differentiate between active and passive gravitational mass. In fact Ohanian asserts that it doesn't make sense to do so. I disagree.

Hint: Before any experiment can be done one must first define the quantities that are being measured. Only after that can a measurement be made.

For a journal articles on active gravitational mass see

Active Gravitational Mass by Charles W. Misner and Peter Putnam
Phys. Rev. 116, 1045 – Published 15 November 1959
https://journals.aps.org/pr/abstract...ysRev.116.1045

A very famous paper on the concept see Negative mass

Negative Mass in General Relativity by Herman Bondi, Rev. Mod. Phys. 29 (3): 423–428.

In this article Bondi talks about the possibilities of these three different masses having different signs and the theoretical concepts which follow.

You can't look for those things which you choose not to think about and ascribe only to blackboards.

When it comes to tachyons the mass is imaginary. Imagine that?
https://en.wikipedia.org/wiki/Tachyon
 Gerald Feinberg proposed that tachyonic particles could be quanta of a quantum field with imaginary mass.

Frankly I'm confused as to your objection of a concept which you don't appear to ever even heard of. Why is that?

Jul 16th 2017, 04:13 PM   #20
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 Originally Posted by oz93666 I don't understand this at all .... Yes newton defined mass as F=ma
That is incorrect. Newton didn't define inertial mass in that way. F = ma is Euler's formulation.

I believe Newton's definition was p = mv. In Principia Mathematica by Isaac Newton, Newton states that p = mv where m = density x volume.

To be precise he says something like (I have to paraphrase since I can't find my darn copy anywhere ... grrrrrr)

"Inert mass is the product of volume and density" and the goes on to say that "quantity of motion is the product of velocity and inert mass"

Quantity of motion was the precursor to momentum. To Newton this was a scalar quantity. It was only later shown that the conserved quantity was the vector p = mv.

 Tags energy, kinetic, mass, object, relation