Two glasses

I tested a glass of water. The more water I put in it, the lower the sound. I then tested two more classes of different shapes. Same result. I then set up four glasses and adjusted levels so I could play the first few measures of Jingle Bells. There is no argument that can stand up to Jingle Bells, no matter how erudite the explanation.

I wrote a comment earlier but don't see it now, so this may be a repeat.

Water is denser than air, which means that there are more particles in the same amount of space. Since there are more particles, they vibrate faster and bump into each other force. Pitch is defined as the frequency of vibration of molecules, and water vibrates faster, so that causes it to have a higher pitch.

Water is used in hydraulics due it's property to distribute the pressure evenly because the glass container water it absurd a lot of energy and this decrease the vibration of the glass as The pitch is dependent upon the frequency or the three number of vibration per second.

The glass is like a string on guitar. And water is like a finger on it.

consider simple harmonic motion .. a spring and a mass .. if you increase the mass, you lower the frequency the water acts as extra mass

alternatively: the resonant cavity is larger for the empty glass .. but this is a minor component .. by far the sound you hear comes from the glass vibrating

This problem has been posted with an incorrect answer and solution, based on the length of the air column in the glass. In that solution the shorter air column yielded higher frequency. It has been concluded that the higher frequency comes from the glass with lots of water. That solution has been incorrect. When a "knife lightly strikes the glass" it excited vibrations in the glass, not in the air contained by the glass. The length of air column is irrelevant to the solution.

What is relevant, instead, is the water mass $m$ involved in the vibrations. When the glass is empty the relevant mass is the mass of the glass and the restoring force is the elasticity of the glass, $k$ . When water is added, the mass is increased and the restoring force is the same (the water does not contribute to the restoring force, because it does not hod its shape). Since the frequency is proportional to $\sqrt{k/m}$ , the frequency of the glass with the water will be lower.

Now you got this right. 😉

I literally just tested it out in the kitchen.

Water increases total mass which lowers vibrational frequency which means lower pitch

An empty bottle produces a lower pitch because there's lots of air in the bottle to vibrate. Adding water to the bottle decreases the amount of airspace, which means there is less air to vibrate. With less air, the vibrations happen more quickly and produce a higher pitch.

Have you ever tried to talk under water? Then you know your voice is muffled this means the water is denser than air also causing it to be a lower pitch.

In water the frequency is way more than in free air, so in water filled glass will produce higher pitch

When you tap it, it creates vibrations in the cup. It's like in a guitar, because water has a higher density, acting like the walls of the guitar. This makes it have less energy. Also, when the sound waves hit the water, that makes the sound waves lose energy, causing it to have a lower pitch.

The case here is if the glass is stroked with a knife. Since the empty glass contains air, the air molecules vibrate freely without borrowing much energy from the vibrating walls of the glasses. The full glass contains water, which is more dense than air. Thus, it takes up more energy from the walls of the glasses to produce the same vibrations an air column did. Since the total energy is same, we can say that the empty glass vibrates with a higher frequency, leading to a higher pitch and the full glass vibrates with a lower frequency, i.e. lower pitch.

Because a sound wave moves faster in the water rather than in the air (because the water is denser), therefore the frequency of the wave crossing the full glass will be higher than the empty one, resulting in a higher pitch.

A lot of you are talking about air pressure, or the resonant frequency of the glass changing due to higher pressure or tension, but those negligible effects that, although they work in the direction of a higher frequency, aren't the main contribution and are mainly irrelevant. The biggest contributing factor is that the length through which the sound wave travels, reflects off the bottom of the glass, and superposes with the original wave, causing a standing wave to form. Shorter length due to added water causes the frequency of the standing wave to be higher. For example, in a trombone, when you pull the rod, it's not the resonant frequency that's changing, or air pressure, but the length through which the wave travels. Here is an animation of this

http://www.walter-fendt.de/html5/phen/standingwavereflection_en.htm

Also, a good Khan Academy video about this for those of you who seem to be really misguided or ignorant and trying to mislead others and need a good review:

https://www.youtube.com/watch?v=BhQUW9s-R8M