4. A block of mass m is placed on a block of mass M as shown in Fig. 2. The coefficient of kinetic friction for all surfaces is μk. Ignore the pulley and the rope. For what value of the horizontal force F will the blocks move at constant speed?


(A) (m+M)gμk
(B) (m+2M)gμk
(C) (2m+M)gμk
(D) (m-M)gμk
(E) (3m+M)gμk

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