Linear Momentum (p = mv)

Momentum is mass in motion: multiply how much material is moving by how fast it moves. A 40,000 kg truck creeping along at 1 m/s carries 40,000 kg·m/s of momentum, while a 100 kg bicycle-and-rider at 10 m/s carries only 1,000 — which is why the slow truck is far harder to stop. Newton called momentum the "quantity of motion" and framed his second law around its rate of change, not around ma.

Momentum's real power is conservation: in any collision or explosion with no outside force, the total momentum before equals the total after. That single principle lets crash investigators reconstruct vehicle speeds from wreckage and lets rocket engineers plan burns — the exhaust thrown backward and the craft pushed forward always balance. In this one-dimensional form, direction simply rides along as the sign of v.