# Confusion over 'what is mass'

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##### Guest
Matter is the amount of stuff but it seems mass is also this. Matter seems always to exist but mass only appears in an accelerating field. It's different from weight but proportional in as much as it can be consistently measured the same by cancelling the accelerating field. So is mass a <b>property</b> of matter & not actually existing sort of like temperature?

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#### nacnud

##### Guest
Mass is the measure of a things inertia, or resistance to change in momentum. Yes, it is a property of a thing.

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#### heyscottie

##### Guest
The "mass" property of matter always exists, whether we are in an accelerating field (such as gravity) or not.<br /><br />There are actually two masses, which are equivalent at a relative rest velocity:<br /><br />1) Gravitational mass. This quantity determines how much the matter puckers space-time, and how strongly it attracts other mass to itself.<br /><br />2) Inertial mass. This quantity determines how difficult it is to accelerate the matter.<br /><br />As I said, these two quantities are exactly equivalent at relative rest velocities, which (I believe) is a mystery that has not yet satisfactorily been solved.<br /><br />At relativisitic speeds, inertial mass, and therefore momentum, increases, but gravitational mass remains the same.

S

##### Guest
Thanks both, I need to cogitate on this before replying.

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#### dan9678

##### Guest
RE: "At relativisitic speeds, inertial mass, and therefore momentum, increases, but gravitational mass remains the same. "<br /><br />Has anyone ever proved this? I'd be very currious to see how.<br /><br /> -Dan-<br />

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#### heyscottie

##### Guest
Well, in particle accelerators, we accelerate charged particles to very sizable fractions of the speed of light. It is easy to measure the inertial mass by seeing how much more difficult it gets to accelerate these particles further. Their inertial mass would approach infinite as they approach the speed of light. They do not, however, turn into gravitational black holes that swallow up the earth.<br /><br />So there is some qualitative proof! It is certain that experiments have been run along these lines to quantitatively measure the effects to show them in line with Relativity.<br /><br />Scott

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##### Guest
I've thought over this mass problem & have started a new topic from the result.<br />It's "Mass doesn't prevent reaching C". Sorry can't link it as IE6 just doesn't work, sent scores of errors to M\$, so I'm using OB1 which doesn't let me copy links.<br />Oh after 18 min it's come up Link

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#### xmo1

##### Guest
Here is a notion put forth in a class:<br />In billiards, when a faster moving white ball hits a stationary red ball, the white ball seems more massive because it doesn't get deflected as much as the red ball. Is that quantitively true? The speedier ball acts as if it has a larger mass. What is mass, and how can it change with speed?<br /><br />According to Newton, mass is what relates force to acceleration.<br />Mass is a measure of how hard a body resists a change in velocity.<br /><br />My question is bound in the following:<br />First, a comment on the illustration:<br />The mass, speed, and angle of incidence are responsible for the amount of force felt by, and the amount of the energy transacted by the two objects. The energy is changed from potential energy to kinetic energy on the receiving ball, and a change of kinetic energy to potential energy on the imparting ball, along with a small release of heat shared by the two balls at the point of incidence. The temperature of the heat is the same for both balls, but the amount of heat dissipation on either ball is a result of the type of material (with relative density and other properties) that each ball is made of, that is, the relative ability of each of the two balls to carry away the resultant heat. Heat will propagate faster through whichever material will allow that to happen quickest. Given identical surfaces and materials the heat transfer will be shared equally between both objects. The direction of travel that results from such a collision is equal to the angle of deflection.<br />True or not?<br /><br />Second, is this gravational mass that I describe below?<br />Mass is the sum of the weights of the components bound in the structure, and is constant for that object unless changed by some outside factor.<br /><br />So I am wrong in stating:<br />Mass does not change with speed. Velocity has no affect on mass at all.<br /><br />We must specify whether the mass is gravational or inertial, because speed and acceleration <div class="Discussion_UserSignature"> <p>DenniSys.com</p> </div>

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