Abstract: By using the effective-theory method，the free-fall problem，the micro-amplitude vibration and

the particle motion in central potential field are investigated respectively.Based on the manifest symmetry that each

of problem possesses，part of degrees of freedom are decoupled from the full theory and thus the corresponding

effective Lagrangians and effective Hamiltonians are derived.These examples slow the superiority of effective-theory

method in studying classical mechanics.

Key words: effective-theory method, symmetry, decoupling