This simple device provides all the necessary features to give an accurate presentation of vertical motion. Two steel balls are machined to have the same mass so they can be interchanged. Insert one ball inside the rod-mounted housing and slide the other over the end of the protruding shaft. When you pull the pin, locking the shaft in the "cocked" position, the spring-loaded shaft slides out of one ball, allowing it to drop, and simultaneously hits the second ball, firing it into flight. Length of sping-loaded shaft has been carefully engineered so both balls are released at the same time.