Since the increase in energy is accompanied by an increase in mass, mass and energy are considered equivalent, expressed by:
$$E=mc^{2}$$
where,
E— energy;
m— mass;
c— speed of light in a vacuum.
As the velocity approaches the speed of light in a vacuum, the velocity cannot significantly increase, and the mass of the body begins to increase, storing energy as additional mass.
The speed of light in a vacuum is a physical constant with a value of:
$$c=299,792,458 \frac{m}{s}$$