Q.
A uniform rod of mass 20 kg and length 5 m leans against a smooth vertical wall making an angle of $60^\circ$ with it. The other end rests on a rough horizontal floor. The friction force that the floor exerts on the rod is: (take $g = 10$ m/s$^2$)
A) 100 N
B) $100\sqrt{3}$ N
C) 200 N
D) $200\sqrt{3}$ N
Explanation
[Solution by Claude - not from source] Friction $f=N_{wall}$. Torque about the foot: $N_{wall}L\cos60^\circ=mg\frac L2\sin60^\circ\Rightarrow N_{wall}=\frac{mg\tan60^\circ}{2}=\frac{200\sqrt3}{2}=100\sqrt3$ N. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.