$①當t<2時,點M在線段AO上,點N在線段BO上$
$\frac{1}{2}(4-2t)(3-t)=\frac{1}{4}$
$解得t_1=\frac{5+\sqrt{2}}{2},t_2=\frac{5-\sqrt{2}}{2}$
$∵t<2$
$∴t=\frac{5-\sqrt{2}}{2}$
$②當2<t<3時,點M在線段OC上,點N在線段$
$BO上 $
$\frac{1}{2}(2t-4)(3-t)=\frac{1}{4}$
$解得t_1=t_2=\frac{5}{2}$
$③當t>3時,點M在線段OC上,點N在線段OD$
$上 $
$\frac{1}{2}(2t-4)(t-3)=\frac{1}{4}$
$解得t_1=\frac{5+\sqrt{2}}{2},t_2=\frac{5-\sqrt{2}}{2}$
$∵t>3$
$∴t=\frac{5+\sqrt{2}}{2}$
$綜上所述,出發(fā)后\frac{\sqrt{5}-2}{2}s或\frac{5}{2}s或\frac{5+\sqrt{2}}{2}s時,$
$△MON的面積為\frac{1}{4}cm^2$