Tutoring physics, relativity may arise. The tutor mentions its application to mass.

Einstein postulated, in his Special Theory of Relativity, that effective mass increases as velocity does, according to

m=m_o(1-v²/c²)^-0.5

where m_o is the rest mass, while c is the speed of light.

The rest mass of an electron is 9.11×10^-31kg. If travelling at 80.0% the speed of light, or 0.800c, the electron’s mass will be

m=9.11×10^-31(1-(0.800c)²/c²)^-0.5

which gives

m=1.52×10^-30kg

or 1.67 times its rest mass.

Source:

Giancoli, Douglas C. Physics, 5th ed. New Jersey: Prentice Hall, 1998.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.