Cut the wood strip into 3 12-inch pieces and 8 6-inch sections.
Set the miter saw at a 45-degree angle and cut the corners on two of the 6-inch pieces so they resemble long trapezoids.
Screw one corner of an angled piece flush against the corner of a 12-inch section, creating a 45-degree piece. Slide an uncut 6-inch section against the extended end of the cut piece. Screw one end into the 12-inch piece and connect the angled section to the upright 6-inch section with wood glue. This should result in a triangle with a short arm coming off the top and a long arm coming out the base.
Create a mirror image of the triangle built in the previous step. These will serve as the base braces for the catapult.
Line the braces 6 inches apart from each other and connect the frame by screwing 6-inch sections into both ends of the 12-inch base arms. Connect the two upright 6-inch arms with another 6-inch piece and screws. Rest the remaining 6-inch piece about halfway up the incline of the angled arms and screw in to brace the ramp.
Drill a small hole approximately 1/2-inch in from one end of the last 12-inch section. The hole should be just large enough to slide the coat hanger wire through.
Measure 3 inches from the opposite end of the 12-inch piece and insert an eye hook. Fold a piece of poster board into a small open box and set on the catapult arm in the 3-inch space between the eye hook and the end of the arm.
Drill a small hole in each of the upright base posts and attach the catapult arm by sliding the coat hanger wire through the arm piece and insert both wire end into the base posts.
Insert a second eye hook into the base of the catapult. Center it on the 6-inch piece that connect the 12-inch sides directly in front of the angled pieces. Create spring tension by connecting a rubber band across both eye hooks and pull back on the catapult arm to release and launch.