Damper is inclined with the direction of motion of mass:

For actual system $F_D$ = c( $\dot{x}$ cos $\alpha$) cos $\alpha$

= $c \dot{x} \ cos^2 \ \alpha$ - - -(1)

For equivalent system

$F_D \ = \ ce \dot{x}$ - - - (2)

From equation (1) and (2)

$c_e …