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We will find a constant of motion with energy units for a relativistic particle moving in a quadratic dissipative medium subjected to a force which depends on the position. Then, we will find the Lagrangian and the Hamiltonian of the equation of motion in a time interval such that the velocity does not change its sign. Finally, we will see that the Lagrangian and Hamiltonian have some problems.

It is well known that the Lagrangian and Hamiltonian approaches for dissipative systems have some problems [

In this paper, we will find a constant of motion, the Lagrangian and the Hamiltonian of the equation of motion of a one-dimensional relativistic particle in a quadratic dissipative medium subjected to a force that depends of the position, in a time interval such that the velocity does not change its sign.

This problem is already solved if we consider the classical second Newtons law [

where m is the mass of the particle, x is the position, F is the force, q is the sign of the velocity and k is the dissipative parameter that can depend on x.

In this case, it is known that the following quantity is a constant of motion:

where v is the velocity, q is the sign of v,

From this constant, the Lagrangian, the momentum and the Hamiltonian of the system are obtained [

Note that since q can be 1 or −1, then there are two possible Lagrangians and Hamiltonians. Note also that in the free particle case, i.e., if

If we consider the relativistic second Newtons law, this problem is solved only in the free particle case and considering

where

In this case, the constant of motion is the following:

From this constant, the Lagrangian, the momentum and the Hamiltonian of the system are also obtained and they have the same problems (see reference [

If we consider the relativistic second Newtons law, the equation of motion of the particle is the following:

On the one hand, this equation can be written as the following system:

On the other hand, a constant of motion of this equation is a function

Using Equations (9) and (10) we arrive to:

We will propose the following quantity as a solution of this equation:

We have that:

Hence we arrive to:

If we consider

in Equation (12) is a constant of motion.

Note that if we take

The Lagrangian of the system can be consistently deduced from the known expression [

Hence we have that:

From this Lagrangian, we can find the momentum by doing

The inverse relation of this equation is given by:

Replacing this expression in Equation (12) we obtain the Hamiltonian:

Note that if we consider

We obtained a constant of motion, the Lagrangian and the Hamiltonian for a one-dimensional relativistic particle in a quadratic dissipative medium subjected to a force that depends of the position, in a time interval such that the velocity does not change its sign. We could generalize Equations (3)-(5) to the relativistic case and we saw that the problems we had in the classic case continue. This suggests that the Hamiltonian approach applied to dissipation problem may bring about incorrect solutions if it is directly applied to quantum mechanics or statistical physics.

FedericoPetrovich, (2015) One Dimensional Relativistic Particle in a Quadratic Dissipative Medium Subjected to a Force That Depends on the Position. Journal of Modern Physics,06,1185-1188. doi: 10.4236/jmp.2015.69122