Homework problem dealing with frictional forces when a box is inside of a truck

[Confusingmeh]

Homework Statement

A 30.0 kg packing case is initially at rest on the floor of a 1500 kg pickup truck. The coefficient of static friction between the case and the truck floor is 0.30 and the coefficient of kinetic friction is 0.20. Before each acceleration given below, the truck is traveling due north at constant speed.
(a) Find the magnitude and direction of the friction force acting on the case when the truck accelerates at 2.01 m/s2 northward.
(b) Find the magnitude and direction of the friction force acting on the case when the truck accelerates at 3.59 m/s2 southward.

Homework Equations

Ff=u(Fn)

The Attempt at a Solution

Ok, so when I first saw this problem I assumed that the only difference between the two frictional forces would be the direction. So I got that part of the question (a is northward, b is southward), and on part a I figured that it was the static friction that was pertinent so i did that to find the friction force and was wrong, so I tried using the kinetic friction and was still wrong. So I moved on to part b and was correct when i used the kinetic friction to find the friction force. *note we enter our answers online so although it told me that my magnitude in part b was right it may not have been the right way to do it.

 

[Sniperman724]

First you would need to figure out how much force the packing case is exerting on the truck by using the formula F=mg, then set it up as a typical free body diagram with the normal force going up, gravity down, and static/kinetic friction on one side and the acceleration and speed of the truck on the other side.

I hope this helps