Physics: Why Water in Car Acceleration is Perpendicular

[drabdallh]

if water in a container in a car and the car accelerates why the surface of the water is perpendicular to the net force of gravity and fictitious force not to the net of normal and gravitational force?

 

[rcgldr]

It's the same net force on the water, the only difference is the frame of reference, such as inside the car (fictitious force, no apparent normal acceleration), versus the ground outside the car (force related to normal acceleration of water).

 

[drabdallh]

you don't get what i mean
imagine the surface of the water
it will be perpendicular to net force of gravity and fictitious force
while the normal force is the actual acting force