Q. A plank with a box on it at one end is gradually raised about the other end. As the angle of inclination with the horizontal reaches 30°, the box starts to slip and slides 4.0 m down the plank in 4.0s. The coefficients of static and kinetic friction between the box and the plank will be, respectively :
A) 0.4 and 0.3
B) 0.6 and 0.6
C) 0.6 and 0.5
D) 0.5 and 0.6
Explanation
[Solution] $\mu_s = \tan 30° \approx 0.58 ≈ 0.6$; $s = \frac{1}{2}at^2 \Rightarrow a = 0.5$ m/s², and $a = g(\sin30° - \mu_k\cos30°) \Rightarrow \mu_k \approx 0.52 ≈ 0.5$. Answer (3).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.