Derivation Challenge:
A force F is exerted on the left hand side of a box of mass m1. Starting with Newton's second law, derive the formulas shown for F32 and F21 (where F32 is the force on box 3 exerted by box 2, and F21 is the force on box 2 exerted by box 1).
The coefficient of kinetic friction μk is the same for all box/ground surfaces. Yes, the coefficient of friction ends up cancelling in your derivation, and yes that is amazing, and yes you must show how it cancels in your derivation. (+3% on Test 3)