A uniform, $4\mathrm{.}5$ kgkg$,$ square, solid wooden gate $2\mathrm{.}0$ mm on each side hangs vertically from a frictionless pivot at the center of its upper edge. A $1\mathrm{.}2$ kgkg raven flying horizontally at $4\mathrm{.}5$ m$/$sm$/$s flies into this door at its center and bounces back at $1\mathrm{.}5$ m$/$sm$/$s in the opposite direction.