Hot drink pitch

[vanadium]

When a hot drink is stirred rapidly and then the spoon is tapped on the bottom of the container, the pitch of the sound made increases. Why?

[Physicsgirl]

the pitch of the sound depends on the length of the thing vibrating to make the sound, broadly speaking. So it could be that the rim at the top of the cup is the bit that vibrates, and when you stir it, the liquid comes up the side and shortens it. Only guessing. But then, this doesn't fit with the fact that even when you stop stirring the pitch increases as the liquid slows down. So maybe it's the air at the top of the cup that's the vibrating bit, and as the liquid slows down, the distance between the middle of the liquid surface and the rim decreases. Could also be that as the air above/around the cup heats up, the pitch of the note propagated through the air changes. It doesn't really happen with cold drinks.
I've done this quite a few times (sad, I know) and am always intrigued. It just keeps getting higher!

[wildwood]

Rapidly revolving liquid in a cup does not put the same pressure on the bottom as when the liquid is not in motion, hence this alters the pitch of the sound waves when the bottom is tapped. Then again, I thought this up all by my little self so it could be totally wrong. Physicsgirl's answer looks better, I'd go for that if I was you.

[mibn2cweus]

The frequency at which a resonant system oscillates is lowered in proportion to its mass. A liquid in contact with the surface of a resonant body contributes its mass through mechanical coupling, increasing the inertial moment and thereby reducing the resonant frequency of the system.

Stirring of the liquid in a vessel increases angular momentum in the liquid throwing it outward. With outward momentum confined by the surrounding vessel the liquid is forced up the side of the container until the force due to momentum is in equilibrium with the downward force of gravity. The resulting increase in the surface area of the vessel in contact with the liquid lowers the resonant frequency of the oscillations induced by the tapping.

As angular momentum dissipates and rotation slows the level of the water at the surface of the container gradually falls reducing the contribution of the liquid to the resonant frequency of the system and the pitch rises proportionately.

However, your probably referring to the hot chocolate effect

[Physicsgirl]

glad to see that I'm not the only one who answers random questions at strange times during the night! Nice answers, you two.

[mibn2cweus]

Perhaps even stranger was me banging around with the pots and pans in the kitchen all night doing research?

Oh the hardships we endure in the interest of science!

[Physicsgirl]

True! I went to the kitchen and set up an experiment with two identical mugs, then tried varying amountsof hot, cold, stirred and unstirred water in each, before making my contribution. It'll make a good question for my A level physicists too. Still, it's a better way to spend time when you can't sleep than watching brain-dead TV!

[Teddio]

The rise in temperature of the ceramic changes it resonant properties.

[ludwig]

It's definately to do with the heat not the stirring. The effect happens with hot water whether you stir it or not. Conversely, cold water in the cup doesn't produce the effect no matter how you stir it.