modifica
{ http://lectureonline.cl.msu.edu/~mmp/kap3/cd052a.htm [physics] [vector]}
{ http://lectureonline.cl.msu.edu/~mmp/kap4/cd082.htm [physics] [vector] [force]}
{ http://lectureonline.cl.msu.edu/~mmp/applist/si/plane.htm [physics] [vector] [force] [inclinedPlane]}
{ http://lectureonline.cl.msu.edu/~mmp/kap4/cd095a.htm [physics] [vector] [force] [inclinedPlane]}