2D motion physics problems like the monkey physics problem

Someone ask: 2D motion physics problems like the monkey physics problem

Caveman know monkey physics problem as trick with falling and moving at same time. Monkey hangs from tree, hunter aims spear. If spear fired straight at monkey, both spear and monkey fall same amount from gravity. So hunter must aim where monkey will be, not where monkey is. Same stone works for ball thrown from cliff, cannonball, or river current plus falling rock: split motion into two parts, sideways and up-down, then think each part alone. Sideways moves steady unless force push it. Up-down gets pulled by gravity. Put parts together, path make curve like spear flying through air. Cave wisdom: 2D motion easy when Caveman break one beast into two simple beasts.