From Scientific American:
Can you turn a cup of water upside-down without the water pouring out? Sounds impossible, right? This project will show you how you can do it using a neat physics trick!
Have you ever gone around a fast curve while riding in a car? Did you notice how it feels like you are being “pushed” to the outside of the curve? You are not actually being pushed—rather, your body “wants” to continue traveling in a straight line. (This is called inertia.) Your seat belt and the friction of the seat against your body pull you along with the car as it turns, however. This force that makes your body turn instead of continuing to go straight is called centripetal force.
So, that works when you go around a horizontal curve in a car. What happens if you go through a vertical curve, such as a roller coaster loop? In a roller coaster all riders are strapped in for extra safety. There are plenty of examples where things go through vertical loops without being strapped in—such as a toy race car or marble on a track. What prevents them from falling down at the top of the loop? The same concepts apply. The toy car has inertia because it is moving, so it “wants” to keep moving in a straight line. The curved track pushes on the car’s tires (providing a centripetal force), however, forcing it to move in a loop instead of going straight. If the car is moving fast enough, it will speed through the loop without ever losing contact with the track. If it is going too slowly, it will not have enough inertia and it will fall before it makes it through the loop.
It turns out that you can also apply this exact same concept to liquids! …