def SDOF_dashpot(M,K,eta):
    """
    Returns the dashpot rate of SDOF oscillator given mass, stiffness and 
    damping ratio inputs
    
    $$ \lambda = 2\zeta\sqrt{KM} $$
    
    ***
    Required:
    
    * `M`, mass (kg)
    
    * `K`, stiffness (N/m)
    
    * `eta`, damping ratio (1.0=critical)
    
    """
    
    return (2 * (K*M)**0.5) * eta