Mass on an accelerating truck with friction

AI Thread Summary
The discussion focuses on a box with a mass of 24.0 kg on an accelerating truck, analyzing the effects of static and kinetic friction. The maximum acceleration of the truck before the box slides is calculated to be 8.23 m/s². Once the box begins to slide, its acceleration is determined to be 6.762 m/s², attributed solely to kinetic friction acting as the horizontal force. The reasoning behind kinetic friction being the only force is clarified, emphasizing the neglect of other forces like air resistance. Understanding these dynamics is crucial for analyzing motion in systems involving friction.
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http://www.smartphysics.com/Content/Media/Images/Mechanics/06/acceleratingtruck.png

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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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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