Question

A passenger in a car is holding an open water bottle when the car stops suddenly. Why does the water in the bottle splash out when the car stops?

It is in static equilibrium.
It has less friction.
It has inertia.
It loses gravity when in motion.

Answer 1

It’s because of inertia. When the car stops, the bottle (and the car) decelerates but the water tends to keep moving forward (Newton’s first law). Relative motion causes the water to surge and splash out.