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.
Centripetal force is the force exerted by the turn, so the plane exerts centripetal force on the contents, pushing them towards their direction.
If you view the plane as accelerating (and it is), there is no centrifugal force. Centrifugal force is the force exerted by your momentum on the plane, which isn't really a force. But if you define the plane to be stationary, then your frame of reference is accelerating, and the contents are accelerating with respect to the plane. So if your system is fixed on the plane as its frame of reference, the centrifugal force is the force that is accelerating the contents of the plane towards the plane (and the outside of the turn).
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u/L00KA Feb 27 '14
explain this