#### benEzra

##### Moderator Emeritus

SO, how much "mass" (in the Einsteinian sense) does a bullet gain due to its velocity?

I'll use a .223 Remington pushing a 55-grain bullet at 3250 fps. That's 3.56394 grams at 990.6 meters per second (and 1.748626 kJ of energy).

The Lorentz equations tell us that gamma for 990.6 meters per second is 1.000000000005459152744. So the bullet gains 1.94561e-11 grams of "relativistic mass" (that's 19.4561 picograms). So, at the muzzle, your bullet "weighs" 55.0000000003 grains.

FWIW, if 19.4561 picograms of matter were converted into energy in a matter-antimatter reaction, 1.748626 kJ of energy would be released--same as the kinetic energy of a bullet that gains that much mass relativistically.

Isn't physics neat?

(Geek mode off.)