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In a, let's say rectangular container, the water surface always aligns itself perpendicular to the direction of net force acting on it. Why exactly does it happen?

(For example when this container is accelerating towards left, the surface of water aligns it self in such a way that the right end of the liquid is at a higher level than left end.)

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  • $\begingroup$ As reckoned by an observer in a frame of reference that is accelerating with the liquid, the system is equivalent to a stationary frame with the fluid at equilibrium, and acted upon a body force per unit volume of magnitude "$\rho a$" acting in the opposite direction of the acceleration. The vector sum of this body force and the gravitational force per unit volume $\rho g$ is equivalent to a pseudo-gravity of $\rho\sqrt{a^2+g^2}$ acting at an angle to the vertical. The surface of the liquid must be perpendicular to this pseudo-gravity vector. $\endgroup$ Oct 13, 2023 at 1:12

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It's because the molecules are slippery. If there were a force along the surface the molecules would slip along and pile up until they can no longer slide in the direction of the force because there are other molecules in the way.

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