Physics/mechanics of musical box

I recently found an old musical box and most of its workings are obvious but one aspect still remains a mystery to me. Rather amusingly I found a lengthy powerpoint discussing the physics of an almost identical box. This however did not help with my question. Below is the item:

The part I am curious about is circled in red. When wound and running, the vertical orientated metal plate spins like a fan and I believe it plays a role in controlling the tempo. I have read this is by air drag, but I am at a loss to imagine the actual physics behind this. Can anyone give me a better insight?

This image shows it from another angle:

Thanks for any help

Reply 1

littlematt

aKarma

I have read this is by air drag, but I am at a loss to imagine the actual physics behind this.

Thanks for any help

Well looking at it won't it create air resistance as it spins? The faster it spins, the more air resistance it creates and so I guess prevents the music going to fast.

Does that make sense? i'm only guessing.

Reply 2

aKarma

OP

11

littlematt

Well looking at it won't it create air resistance as it spins? The faster it spins, the more air resistance it creates and so I guess prevents the music going to fast.

Does that make sense? i'm only guessing.

This is similar to the explanation I already had and I am looking for a more precise explanation of the why and how.

I initially considered the air flow created across the vibrating comb. But this would mean have no benefit and if anything worsen the situation.

I think it, as you say, it is down to the air resistance. I have not covered any air resistance/drag so will have to look it up but I can suppose that at air resistance increases with velocity to a greater proportion than simply linearly. I guess modelled as a flat surface, it would make sense for it to increase to the square of the velocity. This would mean that a larger portion of the power from the spring coil would be required to maintain the spinning blade when the blade spins faster, which occurs when the coils is more wound, and so would to some degree regulate the turning of the cylinder.

I could look it up, but it is more interesting to try and work it out for myself. Any other ideas? Am I thinking along the right lines?

OP

Bumpety Bump

You should add some suitably shaped blutac or something to make it more streamlined and see what effect it has

Reply 5

aKarma

OP

11

MC REN

You should add some suitably shaped blutac or something to make it more streamlined and see what effect it has

Experimental- I like it :P

Quite a cunning solution regardless of if it works. I think it might be more effective to increase the surface area with paper though as blue tack will be significantly heavier.

I'll be back with the findings later as I'm sure there thousands out there wanting to know. :tongue:

Reply 6

Scipio90

Unless it's spinning really fast or the box has a really delicate and smooth mechanism, I can't really see that providing significant air resistance.

Reply 7

littlematt

Scipio90

Unless it's spinning really fast or the box has a really delicate and smooth mechanism, I can't really see that providing significant air resistance.

Perhaps it's just inertia/momentum it provides then? (another version of your experiment?) It looks like its made of brass or something, so perhaps has large mass for it's size?

Reply 8

aKarma

OP

11

Nope it's definitely air resistance; bear in mind the forces involved are minimal and the fan spins impressively fast. I previously couldn't visualise it as I assumed it made it have a constant tempo. This isn't the case, it merely reduced the amount the tempo changes, which is to say it regulates but not very effectively. It does this as the resistance to the fan increases to something to the degree of the square of the fan velocity and has a vague regulating effect on the power going to the cylinder.

I have a vacuum pump(one of those glass domes with a pump to evacuate a decent amount of air) somewhere and if I can find it I will run it in there and it should run must faster once wound than when ran down. Obviously the sound won't be audible/as audible but I will still be able to see the cylinder turn.

Reply 9

littlematt

aKarma

Nope it's definitely air resistance; bear in mind the forces involved are minimal and the fan spins impressively fast. I previously couldn't visualise it as I assumed it made it have a constant tempo. This isn't the case, it merely reduced the amount the tempo changes, which is to say it regulates but not very effectively. It does this as the resistance to the fan increases to something to the degree of the square of the fan velocity and has a vague regulating effect on the power going to the cylinder.

I have a vacuum pump(one of those glass domes with a pump to evacuate a decent amount of air) somewhere and if I can find it I will run it in there and it should run must faster once wound than when ran down. Obviously the sound won't be audible/as audible but I will still be able to see the cylinder turn.

make sure you post your findings! I'm interested to find out now....

Reply 10

aKarma

OP

11

Wooooo, I found the pump and it's a piece of apparatus Faraday would be proud to call his own:

Unfortunately, I'm missing the glass dome but I'm sure that can be substituted. My preliminary tests (read: messing around) were a great success. Using my high tech upside down jam jar as a dome and a tea light, I was able to extinguish it in two draws of the pump, so it seems to be working.

When I have some more time, I'll find a larger dome substitute and try the box as well as anything that might amuse me. I'm thinking a transparent food pot on a rim off blue tack with some weights on top should be sufficiently air tight.

Physics/mechanics of musical box

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