Physics Help Forum What is mass?
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 Apr 6th 2014, 11:35 AM #1 Member     Join Date: Feb 2014 Posts: 48 What is mass? What is mass? Thanks __________________
 Apr 6th 2014, 01:58 PM #2 Physics Team     Join Date: Jun 2010 Location: Naperville, IL USA Posts: 2,261 Mass is a measure of an object's inertia, or in other words its resistance to being accelerated by a given force.
 Apr 7th 2014, 03:33 AM #3 Member     Join Date: Feb 2014 Posts: 48 Ok. What causes this resistance? Thanks __________________
 Apr 7th 2014, 07:02 AM #4 Physics Team     Join Date: Jun 2010 Location: Naperville, IL USA Posts: 2,261 No one knows for sure. In classical mechanics inertia is a property of things, and the relationship between inertia and force is described in Newton's 2nd Law. But current quantum mechanics theory is that a particle called the Higgs Boson may be responsible for other particles having mass. For more info see: http://en.wikipedia.org/wiki/Higgs_boson KNeves likes this.
Dec 11th 2015, 10:51 PM   #5
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 Originally Posted by ChipB Mass is a measure of an object's inertia, or in other words its resistance to being accelerated by a given force.
Is mass not the same thing as weight?
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Dec 12th 2015, 04:31 AM   #6
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 Originally Posted by marklost165 Is mass not the same thing as weight?
No, though weight is closely related to mass. Weight is the force of gravity acting on a mass, and in general is calculated as the object's mass times the local magnitude of acceleration due to gravity. To see the difference, consider that your weight on Earth is greater than your weight if you were to be standing in the Moon, because the acceleration due to gravity is greater on the Earth than the Moon. In SI units mass is measured in Kg, wheras weight is Kg times m/s^2, which is equivalent to Newtons. So whenever you hear someone say that an object weighs, say, 5 Kg, they are mistaken - it's mass may be 5 Kg, but its weight would be 5 Kg x 9.8m/s^2 = 49N (assuming this weight is measured at the Earth's surface).

 Dec 12th 2015, 06:06 AM #7 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 Hi Anonymous, long time no posts. Mass is not only a measure of how an object behaves under acceleration, it is also a measure of how strongly that object distorts space-time to create a gravitational effect. The distortion caused by an individual proton or electron is truly minuscule, and is generally barely even considered, but put several billion, trillion, (etc.) together (as in a planet for example) and it becomes significant. see wikipedia:Equivalence Principle __________________ You have GOT to Laugh !
Dec 22nd 2015, 04:34 PM   #8
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 Originally Posted by MBW Hi Anonymous, long time no posts. Mass is not only a measure of how an object behaves under acceleration, it is also a measure of how strongly that object distorts space-time to create a gravitational effect. The distortion caused by an individual proton or electron is truly minuscule, and is generally barely even considered, but put several billion, trillion, (etc.) together (as in a planet for example) and it becomes significant. see wikipedia:Equivalence Principle
The aspect of an object that resists changes in momentum is called inertial mass. The aspect of an object that acts to generate a gravitational field is called active gravitational mass. The aspect of a body to respond to a gravitational field (i.e. a body accelerates when placed in a gravitational field) is called passive gravitational field. Mach's definition utilizes Newton's third law.

Note: Just because there is active gravitational mass in a region of space and it generates a gravitational field which causes bodies in it to accelerate it doesn't mean that the spacetime is curved. The gravitational field may very well be a uniform gravitational field which is defined as a gravitational field which has no tidal gradients in it. And tidal gradients is just the Newtonian way of saying spacetime curvature. Thinking that Einstein said that gravity is a curvature in spacetime is a common misconception.
Einstein actual defined the gravitational field according to how objects accelerate when placed in a region of space.

Regarding what mass is please see the paper I wrote on it at:
http://arxiv.org/abs/0709.0687

Regarding the way that Einstein defined the gravitational field please see the paper I wrote on it at:
http://arxiv.org/abs/physics/0204044

Newton defined mass in a circular way. He said that the mass of a body equals the density of it times the volume of the body. He then defined momentum as the mass times velocity and then defined force as the time rate of change of momentum.

Mach's definition of mass is the one most people are familiar with. The one most used in relativity for inertial mass is the one used by Weyl and utilizes Newton's conservation of momentum.

See http://www.finedictionary.com/inertial%20mass.html
 The former definition utilizes Weyl’s definition of mass (m ” p/v) while the later utilizes Mach’s definition (m ” F/a). 8 It is Weyl’s definition that is used in relativity on both sides of this debate and each is referred to as inertial mass.

Last edited by Pmb; Dec 22nd 2015 at 04:42 PM.

 Dec 23rd 2015, 07:06 AM #9 Banned   Join Date: Dec 2015 Location: Tintagel,.Cornwall, UK Posts: 48 Mass All very well but, if I dare say it, it does not say a whole lot to me. Rather is it using marthematical jargon to explain mathematics.? In my teens I was trying to catch up on my mathematical skills (abandoned while I studied for a career in medicine) and was boiling my brains on what was called (stand to be corrected) "partial differential equations ?" Faced with a quadratic equation possessed of four roots I rebelled. I asked for help but received from many diferent sources the same answer, word for wword, so that I became convinced that nobody knew a true answee. I have become a true apostle of the creed that " ... if you can't explain in words of your mother tongue then you need further study". Heiroglyphics will not help. Sorry chaps - no offence intended. I would explain Mass as that which exists in tangible form, the body on which forces act and so cause motion. The force of "Gravity" acts continuously on the Mass of my body and strives to produce motion in a "downward" direction with malevolent intention to degrade my dignity. Determination of the Earth's surface beneath my feet to deny that plot is manifested as Weight. Shoot! Ken Green
Dec 23rd 2015, 08:27 AM   #10
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 Originally Posted by kengreen All very well but, if I dare say it, it does not say a whole lot to me. Rather is it using marthematical jargon to explain mathematics.?
Then I don't understand what it is that you were expecting. The concept of mass is a mathematical concept. It's the mathematical description of the inertial and gravitational aspects of matter. Therefore one must necessarily describe it in terms which use math. This is simply how the mathematical description of nature (i.e. physics) works.

 Originally Posted by kengreen I would explain Mass as that which exists in tangible form, the body on which forces act and so cause motion.
What you have in mind is not mass, its matter. Matter is the physical substance and mass is defined as the mathematical description of matter.

 Originally Posted by kengreen The force of "Gravity" acts continuously on the Mass of my body and strives to produce motion in a "downward" direction with malevolent intention to degrade my dignity.
Again, what you have in mind is better phrased as follows:

 Originally Posted by Pmb The force of "Gravity" acts continuously on the matter of my body and strives to produce motion in a "downward" direction with malevolent intention to degrade my dignity.

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