1. The problem statement, all variables and given/known data okay, I know it says be descriptive and all, but I'll present a watered down problem with some insignificant data already calculated by me. my problem is to understand the logic behind the solution, I know the solution, I simply think my textbook is....... false. here goes. box A with a mass of 5 kg is resting ontop of box B with a mass of 20 kg, and box B is resting on a surface the question is how much maximum force can I apply to object B so A remains stacked exactly at the same spot on top of B without moving the static friction between A and B is Fsmax a,b = 40 the static friction between B and the surface is Fsmax b,s = 20 now I simply fail to fathom why the answer isn't simply 60N, because if I apply 60N in a given direction, and 20 of which will be nullified by the static friction between B and surface, and 40 remains which will bring the static friction between B and A to it's max, any further application of force should move A off of B, or so it is in my mind. 2. Relevant equations [tex]\sum[/tex]f = ma(duh) 3. The attempt at a solution I will list the answer to this problem, according to my text book here. the claim is I first calculate f = ma of body A where f is the Fsmax a,b. this will result in a = 40 / 5 = 8, this determines the supposed "max acceleration" so I now calculate f = ma for body B and this new a. F - fsmax a,s = 20 * 8 F = 180N I just cannot fathom that you could apply 180N to object B with the friction between B and surface being only 20, and still object A won't move when it's max static friction with B is only 40... where are the remaining 120N going?? I think this whole calculation is bogus, but I'm probably wrong heh... text books aren't wrong. please, someone. help me understand this.