Hello, I have the answer for part A) but not part B. I attempted part B, but my

Question

Hello,

I have the answer for part A) but not part B.  I attempted part B, but my answer seems wrong to me.  

QUESTION: At a construction site, a small crane is raising 2 boxes of nails on a plank to the roof.  One box has already been opened and is half full, while the other box is new.  The boxes, including the nails, weigh 10 kg & 20 kg, respectively, and are the same size.

A) If the coefficient of static friction is 0.4, draw an FBD for each box of nails and use it to calculate the angle at which each box begins to slide.

--> The answer I got was 21.8 degrees for the angle

PLEASE ANSWER FOLLOWING:

B) If the coefficient of kinetic friction is 0.3, how fast will the boxes accelerate along the plank, once they start to slide?

BE SPECIFIC AND SHOW ALL STEPS

Thanks

Explanation / Answer

2)Assuming that the angle does not change once the boxes start to slide
we now have a lower force resisting the sliding because the coefficient of friction is smaller
The downslope gravity driven force is still F = mgsin(21.8) = m(9.81)(0.371) = 3.643m
and the dynamic friction is now Fd = 0.3mgcos(21.8) = 0.3m(9.81)(0.928) = 2.732m

the difference in these is the remaining accelerating force F = 3.643m - 2.732m = 0.911m

as F = ma
0.911m = ma
the mass is common so it divides out and acceleration is
a = 0.911 m/s/s

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