Hi, While writing the fourier representation for the spring-mass system, we have the equation: (-jωm+B-S/jω).U(jω)=F(jω) with F,U,m,B and S being the force, velocity,mass, damping coefficient and spring constant respectively. From this equation, is it right to infer that : 1) Force lags the velocity as F'(jω)= (-jωm) . U(jω) 2)Force leads velocity as F'(jω)= (jS/ω). U(jω) However, physically the above argument doesnt seem to be correct. As force leads the velocity for a mass system and force lags velocity for a spring system. So could someone please correct me the mistake in my understanding.