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## Homework Statement

At a construction site, a small crane is raising two 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 10kg and 20kg, respectively, and are both the same size.

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

## Homework Equations

N/A

## The Attempt at a Solution

I can figure out the calculations/the angle just fine (I believe the answer is 22 degrees), I just struggle with drawing FBD. I'm confused as to how many forces are acting on the boxes. The obvious one is a force of gravity down, and I'm guessing the normal force would be on a slant (perpendicular to the slant of the plank since it's uneven with the weight). There's the force of friction (Is this heading downward or upward?)

I attempted my drawing here:

The lines are a bit shaky since it was just an attempt, but am I on the right track or did I forget something? Thanks in advance! :D

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