Re: Why is that when you add water to a glass and tap it, the pitch decreases?

Sarah,
You are correct when you suggest that the glass is vibrating to cause the 
sound you hear when the glass is tapped.  The glass vibrates back and forth 
disturbing the air just like a tuning fork does. The waves in the glass 
(and tuning fork) in this case are called transverse waves. If you were to 
fill the glass with something that hardens into a solid, the sound will 
have a higher frequency as more of it is added (once it hardens). However, 
water will not stop the glass from vibrating, as a solid will. The water 
slows the vibration of the glass causing a lower pitch as the glass is 
filled.  The same thing happens to a tuning fork when masses are attached 
near the ends of the fork.  The higher the masses are placed the lower the 
pitch of the sound. 

  Since you are from a teacher's college, I assume this is a point that you 
want to explain to your students.  A simple conceptual explanation is 
always best (at first).  I frequently tell my 2nd-year college physics 
students that if they cannot explain a concept to a 10-year old, then they 
do not understand it.

Les Uhrich

PS. the air inside the glass will also produce longitudinal compression 
waves if you blow across the top, especially if the glass is relatively 
tall and narrow.  In this case, the partially filled glass will, indeed, 
produce a higher pitch as the glass is filled (even with water).  However, 
these are different types of waves. In actuality, both types of waves are 
produced when you strike the glass, which is the reason the sound does not 
sound as pure as that of a tuning fork.