either math or physics problem

This is a question of natural frequency. Every object had a natural frequency, when a sources (sounds) providing a particular frequency which very close with the natural frequency of a glass, it will vibrate and to a point it will resonance and cause the glass to break.
The water in the glass, I think, it will cause the vibration of the glass require higher ampitude frequency to break, but I think, it only a little higher would be enough.

2009-04-02 12:31:03 補充：
Increassing the range of the source(your singers) and the glass would need more energy for the source to reach the glass for its natural frequency, since that the energy for the frequency to reach the glass also reach its natural frequency must be enough to break the glass.

2009-04-02 12:31:07 補充：
That mean, you would need a higher ampitude frequency to do so.