For the description of motion in two dimensions one may use Cartesian coordinates (x(t), y(t)), or plane polar coordinates (r(t), γ(t)), where x(t)=r(t)cos(γ(t)) and y(t)= r(t)sin(γ(t)). Rewrite the kinetic energy T= 1/2 m(x'^2+y'^2) in plane polar coordinates.