Forces involving static friction

[america8371]

Homework Statement

The floor of a railroad flatcar is loaded with loose crates having a coefficient of static friction of 0.25 with the floor. If the train is initially moving at a speed of 48 km/h, in how short a distance can the train be stopped at constant acceleration without causing the crates to slide over the floor?

Homework Equations

f = $$\mu$$ F
F = m a

The Attempt at a Solution

This is my first post so I hope did it correctly. Ok so I first tried to draw a force diagram but this is the first problem that has included a speed into it. I know its probably something simple that I'm just not thinking of but I'm having trouble seeing how I can relate speed to any of the equations in this section. Thanks in advance.

 

[Doc Al]

The speed will come in when you apply kinematics to find the shortest distance. But first you must find the maximum acceleration.

Hint: What force accelerates the boxes? What's the maximum value of acceleration that that force can produce?