hi dymir,
We know that the restoring torque of this system is:
τ=-NAIBsin(θ), (where N is the number of turns in the coil)
This means,
I_M*a ̈=-NAIBsin(θ), (where I_M is the moment of inertia, and a is the angular acceleration)
So for small angles:
I_M*a ̈=-NAIBθ,
Hence the...