I keep seeing posts on teams breaking pins because of the pressure from the rubber bands so I thought I would share the following. While rubber bands do provide the power, teams need to consider velocity and acceleration. Velocity is the rate of displacement of an object and acceleration is the rate of change of velocity of an object.
I worked with my teams a few months ago on prototyping various catapults. The final prototype is able to score two balls from the center black line with one rubber band. It was a good learning exercise and then my teams realized they had to figure out how to draw it back, load it, launch it, and keep it from falling apart when it releases. Still plenty of more learning opportunities. 
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