Finding accelration with forces?

[papi]

Homework Statement

Find the acceleration that the cart in Figure 6-38 must have in order for the cereal box not to fall. Assume that the coefficient of static friction between the cart and the box is 0.40.

** the cereal box is at the front of the wagon (outside the wagon)

Homework Equations

I know fs/N= coeffiecnt or .4 but i am unaware of how you find N or fs, please explain

 

[LowlyPion]

Homework Statement

Find the acceleration that the cart in Figure 6-38 must have in order for the cereal box not to fall. Assume that the coefficient of static friction between the cart and the box is 0.40.

** the cereal box is at the front of the wagon (outside the wagon)

Homework Equations

I know fs/N= coeffiecnt or .4 but i am unaware of how you find N or fs, please explain

If I understand your picture that I can't see, draw a force diagram. You have the m*g down and you have the force from acceleration normal to the side and that force times the u of friction must balance the downward force of gravity.