(a) In Figure 4.21, if the coefficient of static friction between the \(40.0-\mathrm{kg}\) crate and the floor is 0.650 , what is the magnitude of the minimum horizontal force the worker must pull to get the crate moving? (b) If the worker maintains that force once the crate starts to move and the coefficient of kinetic friction between the surfaces is 0.500 , what is the magnitude of the acceleration of the crate?

THINKING IT THROUGH. This situation involves applications of the forces of friction. In (a), the maximum force of static friction must be calculated. In (b), if the worker maintains an applied force of this magnitude after the crate is in motion, there will be an acceleration, since \(f_{\mathrm{k}}