Why Does the Pitch Change When Tapping a Spoon in a Cup of Spinning Coffee?

  • Thread starter garytse86
  • Start date
  • Tags
    Coffee
In summary, the authors discovered that the pitch of the sound emitted from a jug of instant coffee depends on the density of air molecules trapped against the coffee grains.
  • #1
garytse86
311
0
if you stir a cup of coffee, is the velocity at the centre different from the velocity outside?
 
Physics news on Phys.org
  • #3
but is the angular velocity always constant?
 
  • #4
No, hence the qualification of "If the angular velocity is constant".
 
  • #5
but what affects the angular velocity?
 
  • #6
I wouldn't know, I don't drink coffee.
 
  • #7
LOL emmmm...
 
  • #8
Something else: why does the pitch sound different when you tap a spoon to the bottom of a cup with spinning (hot) liquid compared to when the liquid is not spinning?

You can actually hear the pitch change when you first stir it very well, start tapping until the vortex slows down.. it's my dad's favorite scientific experiment ;)
 
  • #9
Monique said:
why does the pitch sound different when you tap a spoon to the bottom of a cup with spinning (hot) liquid compared to when the liquid is not spinning?
My hypothesis is that the spinning liquid rises higher up the sides, thus making more contact with the container. The more liquid a container has, the higher the pitch, right? Well, assuming that I've got the correct relationship, then I would conjecture that the glass with spinning liquid returns a higher pitch than when the liquid finally slows down. Should this turn out to be true, I would draw the tenuous conclusion that the pitch is controlled by how much length of glass (to the brim) is not in contact with the liquid.
 
  • #10
pitch change -coffee or cocoa?

Works much better with cocoa, made with milk.

Isnt it the suspended solids (not the vortex) which make the difference to the resonance? Try stirring up and down so the liquid is moving but NOT creating a vortex.
 
Last edited:
  • #11
Newsflash … important research paper …

Monique said:
Something else: why does the pitch sound different when you tap a spoon to the bottom of a cup with spinning (hot) liquid compared to when the liquid is not spinning?

You can actually hear the pitch change when you first stir it very well, start tapping until the vortex slows down.. it's my dad's favorite scientific experiment ;)

Hi Monique!

A very similar issue was examined in the Proceedings of the Cambridge Philosophical Society, volume 65, page 365 (1967), in an experimental research paper entitled:
On note emitted from a jug while mixing instant coffee.

The authors were Farrell W.E. (whose usual field seems to be plate tectonics), Mackenzie D.P., and Parker R L.

Unfortunately, my library has no volumes before volume 77, but from memory they discovered that the note depends on the density of air molecules trapped against the coffee grains - which changes as you stir it!

Well … it was the sixties … and that sort of thing was important … :smile:
[size=-2](I know this is almost a duplicate of a post in another thread
- but I thought it was important)[/size]​
 

1. How does stirring affect the temperature of a cup of coffee?

Stirring a cup of coffee can actually help to evenly distribute the heat throughout the liquid. This is because the motion of stirring creates friction, which transfers heat energy from the walls of the cup to the center of the liquid. Therefore, stirring can help to raise the overall temperature of the coffee.

2. Why does coffee taste better after stirring?

Stirring a cup of coffee can help to mix together any ingredients, such as sugar or creamer, that may have settled at the bottom of the cup. This ensures that each sip contains the same ratio of ingredients, resulting in a more balanced and flavorful taste.

3. Does the direction of stirring make a difference?

Yes, the direction of stirring can make a difference in the taste of your coffee. Stirring in a circular motion helps to evenly distribute the heat and mix the ingredients, while stirring back and forth can create a whirlpool effect that can enhance the aroma and taste of the coffee.

4. How long should I stir my coffee?

The length of time you stir your coffee depends on personal preference. However, a good rule of thumb is to stir for about 10-15 seconds. This should be enough time to mix the ingredients and raise the temperature of the coffee without over-stirring and causing it to become too frothy.

5. Does stirring affect the amount of caffeine in my coffee?

No, stirring does not affect the amount of caffeine in your coffee. Caffeine is a chemical compound that is dissolved in the liquid and will not be affected by the physical motion of stirring. The amount of caffeine in your coffee will depend on the type and amount of coffee beans used to brew it.

Similar threads

  • Classical Physics
Replies
25
Views
12K
Replies
1
Views
2K
  • Classical Physics
4
Replies
131
Views
4K
  • Other Physics Topics
Replies
3
Views
1K
Replies
2
Views
2K
  • Biology and Chemistry Homework Help
Replies
3
Views
4K
  • Advanced Physics Homework Help
Replies
8
Views
2K
Replies
3
Views
766
Replies
12
Views
1K
Back
Top