Final answer:
The trajectory weapon used to fling rocks was the atlatl, a long stick used to propel a spear or dart.
Step-by-step explanation:
The trajectory weapon used at the time to fling rocks was the atlatl. An atlatl is a long stick used to propel a spear or dart. It functioned as an extension of the arm and added kinetic energy to the projectile, increasing its range. The earliest archeological evidence for the use of atlatls comes from France, where a 17,500-year-old atlatl made of reindeer antler was found.