What you Need

What to do

Fill the glass with water, and then try floating the ball in the centre of the water, try and keep it there for more than 10 seconds.

Now try filling the glass right to the top so it is just about to overflow, see if that is any easier.

What may happen

You should find that it is virtually impossible to float the ball at the centre to start with, there seems to be a force pulling it to the side. But if you overfill the glass the ball floats in the centre easily.

Why does it happen?

Water has a property called surface tension, this means that the surface of water is always trying to shrink, and will pull on objects with a constant force for every mm of the edge to try and achive this shrinkage.

Both glass and ping pong balls are hydrophillic (water loving) so water is attracted to them and climbs up the sides slightly forming meniscuses. This means that the surface tension is acting diagonally downward on the ball.

If the ball gets near the side though the two menicuses merge and the water gets flatter in between. So the surface tension on this side is acting almost horizontally, but on the other side the force is still acting diagonally down. Overall the force will therfore pull the ball to the side.

However if the glass is overfilled the meniscus is curving in the other direction which means that as the ball gets closer to the side the surface tension becomes more vertical, so the overall force is pulling the ball back into the centre.

This is also the reason that if you sprinkle small objects (like breakfast cereal) on the surface of water (or milk) they tend to be attracted to both the sides and each other.

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