A block of mass 0.1 kg is connected to an elastic spring of spring constant $640 \mathrm {~N} \mathrm {~m} ^ { - 1 }$ and oscillates in a damping medium of damping constant $10 ^ { - 2 } \mathrm {~kg} \mathrm {~s} ^ { - 1 }$. The system dissipates its energy gradually. The time taken for its mechanical energy of vibration to drop to half of its initial value, is closest to-\\
(1) 2 s\\
(2) 5 s\\
(3) 3 s\\
(4) 7 s