# Friction - assistance needed

fiestytig
Here's my first question:

A 3.3 kg block is pushed along a horizontal floor by a force F of magnitude 20 N at an angle = -40°. 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.

This problem is adapted from a book problem. The figures in the book are: m=3.5 kg, angle = -40; ck=.25. I worked that one and got answers that agreed with the book (a) 11 N (b) .14 m/s2. Plugging the different number into the same problem does not give me answers that are correct! Here's what I did for the book problem:

(a) I first found T using equation substitution like in problem 6-2. I
used T to find Fn and Fn to find fk. For the book problem, I got 11 N,
which is correct per the book. For the homework assignment, I got 10 N,
which is not correct. I checked my math, and I'm not doing anything
differently. What am I missing?

(b) I drew a diagram to show the different forces and decided there's only
acceleration along the x axis, so I only needed to consider horizontal
forces: F and fk. I calculated Fx = F cos A. I figured a = (Fx - fk)/m.
I got .14 m/s2, which is what the book has. I did the same calculations
for the online assignment and got 1.5, which is not correct. I again
checked my math, and I'm not doing anything differently. But what am I
doing wrong?

Thanks,
the spelling-challenged fiestytig