The Spinning Cup Experiment
Why doesn’t water fall out when you spin a cup? The surprising science of centripetal force
Time: 10–15 minutes
Complexity: Beginner
Setup: Cup, water, string, open space
Overview
The Spinning Cup Experiment is a fun and slightly daring activity that lets you explore the physics of motion using simple household items. Fill a cup with water, swing it in a vertical circle, and—amazingly—the water stays inside instead of spilling out. No lids, no tricks, just physics at work.
This Spinning Cup experiment is perfect for helping kids understand how forces act on objects in motion.
It introduces an important idea called centripetal force—the force that keeps objects moving in a circular path. Whether you’re at home, in a classroom, or outdoors, this activity delivers a memorable mix of excitement and learning.
Materials Required
What you need:
• 1 lightweight plastic cup (paper cups work too)
• Water (about half a cup)
• A strong string (about 1 meter long)
• Tape or a hole punch
• Open outdoor space (important to avoid spills!)
Step-by-Step Instructions
- Attach the string securely to the cup. You can punch two small holes near the rim and tie the string, or tape it firmly.
- Fill the cup halfway with water. Do not overfill.
- Hold the free end of the string tightly in your hand.
- Start by gently swinging the cup in a small circle.
- Gradually increase your speed and swing the cup in a full vertical circle (like a Ferris wheel).
- Observe what happens when the cup goes upside down—the water stays inside!
- Try changing the speed and see what happens if you slow down too much.
The Science Behind the spinning cup Activity
At first glance, it seems like gravity should pull the water out of the cup when it turns upside down. So why doesn’t it fall?
The answer lies in centripetal force. When you spin the cup in a circle, you are constantly pulling it inward using the string. This inward pull creates a force that keeps the cup moving in a circular path. The water inside the cup is also moving in that circle.
As long as the cup is moving fast enough, the water is pushed against the bottom of the cup—even when it’s upside down. This happens because the motion creates a force that acts toward the center of the circle, keeping everything in place.
If you slow down too much, the centripetal force becomes weaker. At that point, gravity takes over, and the water will fall out. That’s why speed matters in this experiment.
Spinning cup activity shows how motion and forces work together. The same principle applies to roller coasters, washing machines during spin cycles, and even planets orbiting the sun.
Pro Tip
Start slowly, but make sure to build up enough speed before attempting a full vertical circle. Practice outdoors first—just in case! Using a lightweight cup helps maintain control while spinning.
Expected Results
When done correctly, the water will remain inside the spinning cup throughout the circular motion—even at the top of the swing. If the cup slows down, you’ll notice the water begin to fall, sometimes splashing out mid-spin.
You can experiment further by changing variables:
• Use more or less water
• Try different string lengths
• Compare fast vs. slow spinning
These variations help demonstrate how speed and force affect motion.
Conclusion
The Spinning Cup Experiment is a simple yet powerful way to explore the science of motion. In just a few minutes, you can see how forces work together to create surprising results. It turns an everyday object into a hands-on lesson in physics.
By understanding centripetal force, you’re uncovering the same principle that keeps satellites in orbit and thrill rides exciting. Science doesn’t always need complicated tools—sometimes, all it takes is a cup, some water, and a good spin
