How do I build a safe mousetrap car?
You make it safe by the cord (hi-strength fishing line maybe) you attach to the crossbar on the mousetrap. The cord and the wheel and axle you attach it to or air propeller or whatever drive mechanism you use will greatly slow the snapping speed of the mousetrap. The whole idea of the drive system you use is to extract all the spring energy from the mousetrap and turn it into propulsion. So the answer to all of this is the mousetrap won’t snap at all if you make a good drive system and a heavy enough car to slow the acceleration a little.
