Physicists in 18th and 19th centuries, developing the newtonian Mechanics, defined the mechanical energy
of a particle (a point body with inertial mass *m*).

In particular, they stated that, if a particle is at the point P of a conservative field,
a scalar function of the position **r** of P can be defined, usually denoted by V(**r**), said *potential*,
and the mechanical energy
of the particle in position **r** and with velocity **v** is given by the sum of two functions:

- the
**kinetic energy K**, which depends on inertial mass*m*and on the velocity**v**; - the
**potential energy U**, which depends on the potential V(**r**) and on a property Q, which can be said generally the*charge*of the particle and which expresses the grade of its sensibility to the field;

Q may be, with respect to the field, the*gravitational charge*i.e. the*gravitational mass*or the*electric charge*or other kind of charge.

If we introduce the quantity **p**, said ** momentum** of the particle, defined as

the (1.1) may be written as

and the (1.3) may be written as

From Newton's laws it follows that the energy of a particle moving in a conservative field is constant.

Because the energy is condensed in the particle, the motion of the particle implies a flow of energy.

For example, the energy of a bullet at the mouth of a gun flows due to the motion of the bullet and when the bullet hits the target, if the collision is anelastic, the energy spreads within the target as heat.

Therefore we can say that the energy can propagate due the motion of one or several particles.