In this work we propose using phase diagrams to explain the dynamical behaviour of simple mechanical
systems. First the motion of the system ##IMG##
[http://ej.iop.org/images/0143-0807/36/3/035033/ejp511562ieqn1.gif] {$x(t)$} is experimentally
measured, and then the derivatives, ##IMG##
[http://ej.iop.org/images/0143-0807/36/3/035033/ejp511562ieqn2.gif] {$v(t)$} and ##IMG##
[http://ej.iop.org/images/0143-0807/36/3/035033/ejp511562ieqn3.gif] {$a(t),$} are obtained from it
and the motion equation ##IMG## [http://ej.iop.org/images/0143-0807/36/3/035033/ejp511562ieqn4.gif]
{$fleft( x,v,a right)=0$} is represented graphically. This idea is applied to the study of a
system with linear viscous drag, explaining the evolution of the system towards the dynamical
equilibrium point corresponding to the limit velocity. The phase diagrams of the viscous drag are
compared with those of the Coulomb drag, wh…