I am having trouble with this problem. I found the answer to a to be 14 N, but the online quiz says it is wrong. Fx-f=0. Fcos(theta)-f=0. f=18cos40=14 N. What am I doing wrong? A 3.5 kg block is pushed along a horizontal floor by a force F of magnitude 18 N at an angle = 40° with the horizontal (Figure 6-20). The coefficient of kinetic friction between the block and floor is 0.25. (a) Calculate the magnitude of the frictional force on the block from the floor. (b) Calculate the magnitude of the block's acceleration acceleration.