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 (mass) to get:

M = E / C^2

However, how can this be? In school they always teach this:

Mass- measure of the amount of matter in a substance

Weight- the force of gravity acting upon the substance

So, using those, how can M = E / C^2?

Mass is the measure of the amount of matter in a substance. Therefore, how can you gainmassas you approach the speed of light? You do not gain matter, so therefore how do you gain mass without gaining matter?

So how would it effect mass? Mass should have no effect....