1. Energy of a particle.

Physicists in 18th and 19th centuries, developing the newtonian Mechanics, defined the mechanical energy Eqn000.gif 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 Eqn000.gif of the particle in position r and with velocity v is given by the sum of two functions:




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 Eqn000.gif 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.