The Independence of Motion Accessory enables Vernier projectile launchers to perform the classic experiment where one ball is dropped as another is projected horizontally. The balls strike the floor simultaneously.

A built-in level makes leveling the accessory fast and easy. One ball is placed in the barrel, which causes a pin to extend. A second, drilled ball is placed on the extended pin. When the first ball is ejected from a projectile launcher, the pin retracts and the second ball falls. Both metal and plastic balls are provided.