Mass on an accelerating truck with friction

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http://www.smartphysics.com/Content/Media/Images/Mechanics/06/acceleratingtruck.png [Broken]

A box rests on top of a flat bed truck. The box has a mass of m = 24.0 kg. The coefficient of static friction between the box and truck is μs = 0.84 and the coefficient of kinetic friction between the box and truck is μk = 0.69.

3) What is the maximum acceleration the truck can have before the box begins to slide?
The answer is 8.23 m/s^2

4) Now the acceleration of the truck remains at that value, and the box begins to slide. What is the acceleration of the box?

The answer is 6.762
I did ukmg=ma but i don't understand why kinetic friction is the only force acting on the box.
 
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  • #2
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Kinetic friction is the only horizontal force (as you neglect air resistance and so on). Which other forces do you expect?
 

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