The Laziness Principle

Newton's First Law has a brutally honest name in physics: the law of inertia.

When you slam the brakes, the car slows down but your coffee keeps moving forward at the original speed. The cup did not "fly forward" — the car decelerated while the coffee carried on. That seatbelt exists because of inertia. Your 75 kg body at highway speed carries the momentum of a charging bull.