v=2π/T×√(A²-y²)
v=2π/π √(10²-8²)
=2×6=12cm/s.

Acceleration in simple harmonic motion is
a=ω² y
At y_max=A, a_max=ω² A
Acceleration is maximum at the position of maximum displacement.

v_max=ωA, a_max=ω² A
ω=a_max/v_max = 4/2
ω=2rad/sec.