\[
\boxed{
 \left\{ {\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}x \left( t \right) =-{\frac {C \left( y \left( t \right)  \right) f \left( \sqrt { \left( {\frac {\rm d}{{\rm d}t}}y \left( t \right)  \right) ^{2}} \right) {\frac {\rm d}{{\rm d}t}}x \left( t \right) }{\sqrt { \left( {\frac {\rm d}{{\rm d}t}}y \left( t \right)  \right) ^{2}}}},{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}y \left( t \right) =-{\frac {C \left( y \left( t \right)  \right) f \left( \sqrt { \left( {\frac {\rm d}{{\rm d}t}}y \left( t \right)  \right) ^{2}} \right) {\frac {\rm d}{{\rm d}t}}y \left( t \right) }{\sqrt { \left( {\frac {\rm d}{{\rm d}t}}y \left( t \right)  \right) ^{2}}}}-g \right\} }
\]