Weird Science: Ping-Pong Balance

HONOLULU(HawaiiNewsNow)- It's time for another weird science experiment. Dr. V is going to show us what happens when try to center ping-pong balls in water.

Floating a Ping Pong Ball

Things you need:

  • Cup
  • Water
  • Ping pong ball

The experiment:

Fill the glass with water about three fourths full. Place the ping pong ball in the cup and try floating the ball in the center of the water and see if you're able to keep it in the center for 10 seconds. Are you able to? Now fill the cup all the way up to the top so that the water is just about to overflow and actually bulging over the top of the cup. Now try keeping the ball in the center of the cup. Can you do this? How about pushing the ball to the side of the cup? What happens?

How does it work?

When your cup is not quite full, it is almost impossible to keep the ball in the center of the cup. It's like the ball is getting pulled to the sides of the cup. But when you overfill the cup, the ball floats easily in the center, and now you can't really get it to stay on the side of the cup. When the cup is not full, the surface tension in the cup causes a concave (curving inward) surface. Surface tension means the surface of the water is always trying to shrink and will pull on objects with a constant force. Because the surface of the cup is hydrophilic (water loving), the water rises up the sides of the cup. The ping pong ball is buoyant, it floats, and it seeks the highest surface. When the water in the cup is not filled to the top, the ball clings to the sides of the cup where the water is the highest. When you overfill the cup, you create a convex surface (curving outward), almost like a bubble. Now the highest point on the water is in the middle of the cup, which is where the ball tends to stay.

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