A: Most engineers (and I do the same) implicitly apply the gc conversion without writing it out when working in the English unit system — if the mass is 200 lbm, the equivalent weight force is 200 lbf at standard gravity. I probably didn't state that explicitly in the solution video, but the conversion is embedded. Go ahead and make that assumption in practice: 1 lbm of mass exerts 1 lbf of gravitational force at sea level.