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## Main Question or Discussion Point

Perhaps naively I assumed that for a relativistic particle the product of mass times velocity would be the same in both Newtonian and Einsteinian mechanics. The simplistic thinking was the mass increase in relativistic dynamics would balance out the non-real superluminal velocity of a particle in Newtonian dynamics. I've assumed that my entire life.

So I went to this online calculator below which I presumed gives the final velocity of a particle (electron) accelerated through a particular voltage the user can select. Much to my surprise the expected equality of momentum for Newtonian and Relativistic mechanics wasn't true. I selected "2000", or 2 million volts in the box. In the relativistic case the electron's momentum then comes out to 131.37 x 10^-23, while for the Newtonian case it comes out to 76.4 x 10^-23, approaching half as much.

I'm frankly confused. My intuition was clearly wrong. Something is evidently not being taken into account.

https://www.ou.edu/research/electron/bmz5364/calc-kv.html

So I went to this online calculator below which I presumed gives the final velocity of a particle (electron) accelerated through a particular voltage the user can select. Much to my surprise the expected equality of momentum for Newtonian and Relativistic mechanics wasn't true. I selected "2000", or 2 million volts in the box. In the relativistic case the electron's momentum then comes out to 131.37 x 10^-23, while for the Newtonian case it comes out to 76.4 x 10^-23, approaching half as much.

I'm frankly confused. My intuition was clearly wrong. Something is evidently not being taken into account.

https://www.ou.edu/research/electron/bmz5364/calc-kv.html