The slingshot car is an excellent introductory project to illustrate how potential energy is converted into kinetic energy, and the importance of low friction.
Just be sure to watch your feet as cars go racing across the floor!
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Tips and Troubleshooting
If the slingshot car is spinning out of control, then...
Make sure the wheels are attached parallel to each other.
Check if there is anything that's interfering with the rotation of the wheels, such as loose bits of hot glue or tape, or the frame of the car rubbing against the wheel.