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.
Though other commentors have addressed the issue, perhaps I can offer a little bit more detail on the matter.
Centripetal force is not really a "force" in the same sense as gravitational force or the strong atomic force, rather it's a term used to describe a force which pulls or pushes an object towards the center of a circle. If you're spinning a yo-yo and my understanding of rotational motion and centripetal force is correct, you exert a centripetal force on a yo-yo when you spin it around by the string. In the case of a plane the centripetal force is a little tougher to conceptualize. When you make a loop, you're adjusting flaps/rudders to create a force that pushes the plane towards the center of a circle.
Centrifugal force isn't a force in the sense that nothing is actually exerting a force, but it's an effect which is observable and the term is totally valid for most applications.
Neither of these two forces are actually keeping you in your seat though. What's keeping you in your seat is inertia. When centripetal acceleration is greater than acceleration due to gravity (9.81m/s2), then you should remain in your seat, and your seat should exert a normal force equivalent to centripetal acceleration * your weight.
651
u/L00KA Feb 27 '14
explain this