If \(\theta\) is equal to \(\alpha\) at the time the box is just about to slide down the plank, the coefficient of static friction between the plank and box is
sin\(\alpha\)
cos\(\alpha\)
tan\(\alpha\)
sec\(\alpha\)
cot\(\alpha\)
解释
\(\mu = \frac{F}{R} = \frac{W sin \theta}{W cos q}\)
