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But by definition, mass is the measure of the number of atoms, particles, matter, etc. in an object.

So either they need to stop teaching the wrong definition of mass....or they need to change...

that would increase weight, not mass...

It is common scientific theory that when you approach the speed of light, you gain mass.

This is supported by Eintein's theory:

E = MC^2

which using simple algebra, you can isolate M...