Mechanics > Kinematics > Motion in 2-D DCS# 1D60.10b
AIR TRACK PROJECTILE CART

APPARATUS
air track
 201A
cart with tub, spring-loaded pencil launcher, and push-pin trigger
 202-13-C4
short pencil or small ball

DESCRIPTION
Gently set the cart in motion.  When the string becomes taut, the push pin releases the spring-loaded pencil, which is launched vertically and drops back into the tub on the moving cart. 
The Pasco projectile cart is a more reliable version of this demonstration.
NOTES
Make sure air track is level in both directions.