Which of the following statements are TRUE for damped vibrations?

$$P.$$ For a system having critical damping, the value of damping ratio is unity and system does not undergo a vibratory motion.
$$Q.$$ Logarithmic decrement method is used to determine the amount of damping in a physical system.
$$R.$$ In case of damping due to dry friction between moving surfaces resisting force of constant magnitude acts opposite to the relative motion.
$$S.$$ For the case of viscous damping, drag force is directly proportional to the square of relative velocity.

In a spring-mass system the mass is m and the spring constant is $$k$$. The critical damping coefficient of the system is $$0.1$$ kg/s. In another spring mass system, the mass is $$2m$$ and the spring constant is $$8k$$. The critical damping coefficient (in kg/s) of this system is __________

A point mass is executing simple harmonic motion with an amplitude of $$10$$ mm and frequency of $$4$$ Hz. The maximum acceleration (m/s²) of the mass is _________.

Consider a single degree-of-freedom system with viscous damping excited by a harmonic force. At resonance, the phase angle (in degree) of the displacement with respect to the exciting force is