Dear all, Can anyone explain the the phase lag between force and displacement in single degree of freedom forced vibration with viscosity damping? The response of phase angle derived by mechanical vibration as below ( assuming force is excited harmonically) : Phi(ω) = arctan( C*ω / ( K - M*ω^2 ) ) Where Phi is the phase angle function, C is damping, K is stiffness, M is mass, and ω is angular frequency. From the function above we know that as C is not zero, the phase angle varies with angular frequency, but as C is set to be zero, the phase angle will be constant until K - M*ω^2 equals to zero ( the frequency also known as resonance frequency ). My question is how to explain the phase angle varies with frequency by a physical view ? Thanks for you to think about this problem and every suggestion will be appreciated.