When a car accelerates, the surface of the water in a container aligns perpendicular to the net force, which includes both gravity and the fictitious force due to acceleration. This occurs because the water experiences different forces depending on the frame of reference; inside the car, the fictitious force is perceived, while outside, the normal force and gravitational force are relevant. The net force acting on the water remains constant, but the perception of forces changes based on the observer's position. The water's surface reflects the balance of these forces, illustrating the effects of acceleration in a non-inertial frame. Understanding this concept clarifies why the water's surface behaves as it does during acceleration.
