  The virial theorem is considered for a system of randomly moving particles
that are tightly bound to each other by the gravitational and electromagnetic
fields, acceleration field and pressure field. The kinetic energy of the
particles of this system is estimated by three methods, and the ratio of the
kinetic energy to the absolute value of the energy of forces, binding the
particles, is determined, which is approximately equal to 0.6 . For simple
systems in classical mechanics, this ratio equals 0.5 . The difference between
these ratios arises by the consideration of the pressure field and acceleration
field inside the bodies, which make additional contribution to the acceleration
of the particles. It is found that the total time derivative of the system's
virial is not equal to zero, as is assumed in classical mechanics for systems
with potential fields. This is due to the fact that although the partial time
derivative of the virial for stationary systems tends to zero, but in real
bodies the virial also depends on the coordinates and the convective derivative
of the virial, as part of the total time derivative inside the body, is not
equal to zero. It is shown that the convective derivative is also necessary for
correct description of the equations of motion of particles.
