I just downloaded creo parametric 3.0 trial version, and i don't have important features like gravity, force motors, etc. Could you please help me out.

If I remember correctly trail version of Creo contains modules for part/assembly/drawing/sheetmetal design and basic mechanism design. Things you're writing about (gravity, force motors) belong to additional advanced module Mechanism Dynamic Option (which is sold as separate license) and are simply not available in trial version of Creo.