#### Comment Preferences

• ##### "relativistic mass" is an attempt(0+ / 0-)

to keep some of the mathematics of Newtonian Mechanics intact while using relativistic mathematics.

E.g., the relativistic momentum equation is mass times velocity divided by the square root of a velocity dependent term.  This denominator is the square root of the quantity (1 - speed squared/speed of light squared).  This denominator approaches zero as the speed approaches the speed of light.

Note that mass times velocity is the expression for the momentum of objects with mass in Newtonian Mechanics.  It is easy to think of the denominator of the relativistic expression as dividing into just the mass.  Then define a relativistic mass as the so-called rest mass (mass at zero relative speed) divided by the new term.  This so-called relativistic mass then approaches infinity as the speed approaches the speed of light.

But why should the velocity be ignored in the momentum and only the mass considered?  The speed, which is the amount of the velocity (doesn't include direction), is in the denominator.  It would be really odd to make the change based on speed but drop the velocity from the denominator, but that is what is often done.

Really no one is going to get hurt if you think of mass as something that increases with speed.  You won't be able to get a job in a theory group at CERN, but otherwise, you'll be just fine.

"Trust only those who doubt" Lu Xun

[ Parent ]

• ##### I quote Einstein, when he teaches about(0+ / 0-)

a body that moves with speed v and acquires additional energy E_0 from radiation:

Thus, the body now has the same energy as a moving body of mass \$m+E_0/c^2\$.  We can say, therefore, that if a body acquires energy E_0, its mass increases by E_0/c^2; thus, the inertial mass of a body is not constant, but varies according to its energy.

So, you may well be right that Einstein wouldn't be able to get a job at CERN, but not for the reasons you think.

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