Impulse, Momentum & Rotation

Worked: coupling rail cars

An 80,000 lb car rolling at 3 ft/s couples with a 60,000 lb car at rest. Momentum is conserved and g cancels: v = 80,000 × 3/140,000 = 1.71 ft/s.

Energy is not conserved. Before: ½ × (80,000/32.2) × 3² = 11,180 ft-lb. After: ½ × (140,000/32.2) × 1.71² = 6,390 ft-lb. The coupling dissipates 43% of the kinetic energy, exactly the struck car’s share of the total mass, 60/140.