Question

If you slide a Styrofoam cup filled with water across a finished wooden surface at about 10 cm/s, you can see droplets of water shoot upward to about 20 cm. Why does this spouting occur? Justify answer.

Answer 1

I think I've sort of seen this before with foam cups. There's something about the nature of the contact between the cup and the surface which, i THINK produces pulses of vibrations in the cup as you slide it. I seem to remember noticing this - it didn't seem to be the usual "friction push" or whatever it would be called.
If this vibration is transferred to the water, then the water may, among other things vibrate and splash.
I also seem to remember that these foam cups are "peculiarly top heavy", whatever that means.

Answer 2

To my experience, this resembles the "vibration" of other objects when brushed against a surface. In this case, though the apparent vibration is subtle, its consequences are magnified, probably due to the material.