Science Buddies: "Ask an Expert"

For our Physics class we are required to construct a catapult car. What the car has to accomplish is to travel at least 2 m but no more than 3 m and stop. After that is accomplished you are to launch a ping pong ball towards a target (cup/bucket). The target is 8 m from the start line. I am confused on how to construct a catapult that is self activated once the car has stopped. Any suggestions would be very appreciated.

One other thing I failed to mention was that the only source of energy that can propel either the car or the catapult are rubberbands or springs.

A quick guess: if your car is propelled by means of unwinding a rubber band, the final step of unwinding can release the catapult. In other words, the rubber band, while under tension, holds back your trigger.

Try searching the web using combinations of keywords like 'catapult car mousetrap science project'. There are plenty of projects out there involving either rubber band driven cars or catapults. I haven't seen one yet that combines both though.