Block A in the figure weighs 1.90 N and block B weighs 4. The coefficient of kinetic friction between all surfaces is 0.

Block A in the figure weighs 1.90 N and block B weighs 4.00 N. The coefficient of kinetic friction between all surfaces is 0.300. Find the magnitude of the horizontal force vec F_a necessary to drag block B to the left at a constant speed of 6.50 cm/s if A rests on B and moves with it (figure a).Find the magnitude of the horizontal force vec F_b necessary to drag block B to the left at a constant speed of 6.50 cm/s if A is held at rest by a string (figure b).In part (A), what is the friction force on block A?