A Polystyrene Cup A polystyrene cup, full of water
A carpet

A carpet - office type carpets definitely work well. Make sure it doesn't matter if it gets wet...


Pushing the cupPush the cup across the carpet, slowly, do you notice anything about the cup?

Speed up your pushing until you see some strange effects


Disturbed water 1 Disturbed water 2
As you start the water will start to vibrate It will form waves of different shapes and patterns
Disturbed Water 3 Fountain
It will get more and more disturbed as you speed up. Until it forms spouts fountaining out of the cup.

Here is a video of the effect.


As you push the cup along the carpet it will tend to stick and slip, and stick and slip. 

Stick and Slip

 This will start to vibrate the water creating waves leaving the front and back fo the cup.

Waves from ends of the cup More Waves
This will create a sreies of circularish waves from the edges. The sloshing will tend to distort the cup producing more waves from the sides of the cup.

 These waves then get reflected and interact with one another forming a very disturbed surface.

If in a region of this surface you get a circle of wave converging on a point, this wave is going to get higher and higher as all the energy from the circle of waves is concentrated.

Converging waves 1 Converging Waves 2 Converging Waves 3

In fact as the waves crash into one another at the centre their kinetic energy will tend to get directed upwards into a great big spike.

Colliding waves A spike

If this spike is long enough the end can break away as a droplet and fly off and out of the cup.

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