  We propose a scheme for detecting time-varying weak forces using quantum
probe consisting of single spin and quantum oscillator under the effect of
collective dissipation. We study the force estimation in the steady-state
regime where the information of the force is extracted by measuring observable
of the oscillator such as quadrature and mean phonon excitation. We quantify
the force sensitivity in terms of quantum Fisher information and show that it
diverges approaching the critical spin-boson coupling making the system
sensitive to very small force perturbation. We show that close to the critical
coupling the measurement of the oscillator quadrature is optimal in a sense
that saturates the fundamental Cramer-Rao bound. Furthermore, we study the
force estimation in the presence of phonon squeezing and show that it can
significantly improve the sensitivity reaching minimal detectable force of
order of xN (1${\rm xN=10^{-27}{\rm N}}$).
