The system is pushed horizontally by force F. All surfaces are smooth except the vertical contacts of block B (mass m) with blocks A and C (each mass 2m). The coefficient of friction at those contacts is μ. Find the minimum F that prevents B from sliding downward.

Question

G1 Physics · Accepted Answer

All three blocks have the same horizontal acceleration a = F/(5m). The two horizontal normal forces on B together provide enough vertical friction to support mg. At limiting condition μ(N₁+N₂)=mg. Using the contact-force relations from the common horizontal acceleration gives Fₘᵢₙ = (5/2μ)mg.