I thought about the spring pendulum. Displacement and amplitude s0 to be exactly. In my consideration the spring pendulum forces both gravitation and resilience. So this is my equation:
Fg = Fr
m*g = D*s => s = m*g / D.
That would be the displacement of the spring pendulum.
To calculate the amplitude s0, I thought about the potential relationships. In my consideration the total energy is the same to all kinds of moments in terms of the displacement of the spring pendulum:
E = m*v².
In my view the velocity v is the maximum one, vm. The maximum velocity can be calculate by the maximum displacement sm which is equal to the displacement s at the beginning and the circuit frequency, omega:
vm = s*omega; omega = 2*pi/T
So, the calculation of total energy is: E = m*(s*2*pi/T)². And when I get the total energy, I'm able to calculate the amplitude s0 which kinetic energy is 0 (due to the moment t = 0). In my consideration the total energy consists of elongation. That's why my equation is:
E = 1/2*D*s0² => s0 = square root of 2*E/D
But if I'm consider the amplitude s0 is the maximum one, and the maximum is the displacement at the beginning, my calculation for displacement must be equivalent to the calculation for maximum amplitude:
s = s0 = m*g/D = square root of 2*E/D.
Am I right with all my considerations?