Suppose a body whose **resting mass** will be symbolized by **m _{0}**, is at rest in relation to the ground. If this mass is set in motion and acquires a velocity

**v**, it can be shown that in this situation the mass

**m**of the body will be given by:

Being **m** The** relativistic mass**.

Similar to the items already studied, the denominator of the above expression is also less than 1, so the moving body mass is greater than the resting body mass.

Obviously, the increase in mass does not imply an increase in the amount of particles that make up the stone, but an increase in its inertia. This means that if the body is in accelerated rectilinear motion and under the action of a constant resulting force, the acceleration will not be constant: it will slow down as the speed increases.