Key concepts
Centripetal force

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! If you fill a cup with water and tie it to a string, you can spin the cup around in a horizontal or vertical circle. As long as you spin it fast enough, the water will not spill out! Still don't believe it? Try the procedure below to prove it to yourself!


  • Paper or plastic cup
  • String
  • Scissors
  • Water


  • Find an outdoor area where it is okay if you spill a little water. Make sure there are no people or objects nearby that you could hit when twirling the cup.


  • Carefully poke (or have an adult poke) two holes opposite each other near the top rim of the cup.
  • Cut a piece of string about as long as you are tall.
  • Tightly tie one end of the string to each hole in the cup to form a long handle.
  • Fill the cup about halfway with water.
  • Go outside to an area where it's okay if you spill a little water and where you will not hit any nearby people or objects while twirling the cup.
  • Start twirling the cup over your head in a horizontal circle (so the cup moves parallel to the ground). How fast do you need to spin it to keep the water in the cup?
  • Gradually transition to twirling the cup in a vertical circle. Does the water stay in the cup, even at the top of the loop?
  • If the water spills out of your cup, refill it and try again. Make sure you twirl the cup faster this time. You can also fill it with less water if you continue having trouble.
  • Extra: Try the activity with solid objects instead of liquid. What happens if you put a toy in the cup?
  • Extra: Try changing the length of the string. Does making the string longer (or shorter) make it easier or harder to keep the water in the cup?
  • Extra: Try changing the amount of water in the cup. Does adding more water to the cup make it easier or harder to prevent the water from spilling?

Observations and results
As long as you only fill the cup about halfway, you should be able to twirl the cup in a horizontal circle and then transition to a vertical one without spilling any water. It can be difficult to go directly to a vertical circle because the cup might not be going fast enough on the first loop to prevent water from spilling out. If you fill the cup with too much water, it can also be difficult to prevent some spillage.

More to explore
Centripetal Force, from Science Buddies
Spinning Science: Centripetal Force Using Marbles in Jell-O, from Scientific American
Swiveling Science: Applying Physics to Hula-Hooping, from Scientific American
Centripetal Force, from Hyperphysics
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies