A plank bearing a box is gradually inclined. The box starts sliding at 30° and then travels 4.0 m down the plank in 4.0 s from rest. Find coefficients of static and kinetic friction, respectively.

Question

G1 Physics · Accepted Answer

At limiting equilibrium μₛ=tan30°≈0.577≈0.6. Down the incline, s=½at² gives a=2×4/16=0.5 m s⁻². Use a=g(sin30°-μₖcos30°), so μₖ=[5-0.5]/(10×0.866)≈0.52≈0.5. Answer: B.