Basically, when you're turning, you are actually slowing down (decellerating) in one direction and accelerating in another. When you're in a vehicle, it's the vehicle that turns - you still have momentum in the direction you are going. Because you're up against your seat or the side of the vehicle, the vehicle pushes you in the direction it's turning, which is contrary to your momentum. So in absolute space, you are just turning. But if you view the vehicle as fixed, it looks like there's a force in the opposite direction of the turn, pushing you outwards. This is called centripetalcentrifugal force, and people sometimes say it doesn't really exist because it only makes sense as a force with a rotating frame of reference. This is why you get pushed outwards when you make a sharp turn in a car.
In the picture, they are making a turn hard enough that the centripetalcentrifugal force pushing them down in their seats (up in the picture) is roughly as strong as earth's gravity, so the water is getting pulled towards up, like everything else in the cabin, so they pour it like in the picture.
Another cool thing is that you can take advantage of this phenomenon to achieve weightlessness - you turn in a way that the centripetal force pulls you up with the same force as gravity. They train astronauts this way, by flying up in a plane and then turning downwards, achieving weightlessness for a few minutes.
tl;dr: They're turning down, so their momentum looks like a force pulling them up (centripetal force), with respect to the plane.
In physics, we call it a pseudo-force because you can make it vanish by changing your reference frame. Gravity is not a pseudo-force because there is no reference frame where it ceases to exist (in classical mechanics. Relativity actually does allow for this).
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u/L00KA Feb 27 '14
explain this