Video narration
Why do you lurch forward when a bus stops, or lean back when it starts moving? It can feel as if something pushes you. But does moving require a constant push? On a smooth ride, the bus and your body move together. When the bus brakes, your body keeps moving forward; when it starts, your body tends to stay still. When it turns, your body keeps going straight. This is Newton's first law: an object at rest stays at rest, and an object moving keeps the same speed in a straight line, unless an unbalanced force changes it. That resistance to changing motion is inertia. It is not an extra shove. The bus changes underneath you first. Then the seat, floor, or seat belt exerts the force that changes your motion. That is why you lurch: your body keeps its motion while the bus changes speed or direction.