A Tale of Momentum & Inertia

On a small island near the sea, on a bright and sunny day, a tremendous roar echoed through the jungle. In the distant mountains, a massive rock giant was strenuously piling heavy stones atop a mountain peak. The mountain grew higher and higher, towering majestically. Finally securing the colossal rock, the rock giant turned to leave, only for the stones behind him to suddenly loosen and begin rolling down one side of the slope. At the end of the descent, along the edge of the sea, stood a bustling city. If the giant rock were to roll over it, certain catastrophe would follow. Seeing the danger, the rock giant hurriedly rushed over, and at the critical moment, managed to brace against the rock. However, his heel struck the town's clock tower, causing it to collapse. 　　An unexpected outcome ensued...