Static Friction- Train stops, boxes move?

[battlebball]

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 = ma
f(s)max= u(s)N
final v^2-init. v^2 = 2a*s(distance)
Newton's second law?

The Attempt at a Solution

I've been cracking my skull on this one... I search and found this https://www.physicsforums.com/showthread.php?t=186380 however, I couldn't figure it out.
Is the maximum deceleration -.25g? But when I plug that into: 0-13.33=2*a*s => -13.33/(-2*.25*9.8) = 2.72m and obviously something is wrong... Thanks in advance.

 

[ehild]

Homework Equations

F = ma
f(s)max= u(s)N
final v^2-init. v^2 = 2a*s(distance)
Newton's second law?
Is the maximum deceleration -.25g?

But when I plug that into: 0-13.33=2*a*s => -13.33/(-2*.25*9.8) = 2.72m and obviously something is wrong...

You forgot to square the speed v=13.33 m/s.


ehild