Video narration
Why does water in a truck surge toward the back when the truck pulls away? A cup of water in a car does it too: as you pull away, water lurches backward. At first, it sloshes in waves. But the water isn't simply being dragged backward. It resists being sped forward, so inside the truck it seems to feel a backward push. Gravity, the downward pull, acts at the same time. From the road, pressure from the container helps accelerate the water forward. Together, that force balance gives the settled surface a flat slope, not a level one. That's different from the brief waves at the start, which fade as the water settles. So as the truck accelerates forward, the water settles higher at the back and lower at the front-like water in a cup when a car pulls away.