cola

[han]

You have two identical cans of cola (be it pepsi, coca cola or any other brand, as long as they are both of the same brand).

 

You shake one of them. Then, you place both of them on a smooth hillside and let them roll down. Which can will go fastest?

[Gerben42]

Interesting problem. In my opinion cola is only good for physics problems and things like that (people DRINK that stuff?) but I think this is the solution:

 

 

After shaking you have seperated some gas from the liquid, therefore the shaken can will lose some rolling energy to sloshing inside the can and will roll down slower

 

[helene_t]

Not sure what influence the shaking has on the viscocity of cola, but the expansion leads to more air resistance which means that it must be slower than the non-shaked one.

[pclayton]

But won't the increased pressure on the can from the gas makes the can more cylindrical, and move quicker?

[benlessard]

Inner movement is loss of energy for the full system. A can of sweet peas will go slower then a a empty can or then a full can of tomato paste because the individual peas in it move in all directions. So i guess its the same for Colas shaking will activate the bubble so there will be more bubbles movement so i think it will go slower.

[Winstonm]

Shaken. Not stirred?

[jdonn]

I feel like the shaken can will go faster, for the same reason a full tire is better than a tire that is a little low on air.

 

I would not be surprised to find out I was wrong. Benlessard's explanation looks pretty convincing, I must admit.

[matmat]

i suck at this stuff...

 

1) the can will become stiffer, as the released gas will increase the internal pressure

2) the density and volume of the liquid inside the can will not change significantly, but the pressure on the liquid will be greater, so it will actually be more difficult for the liquid to slosh about.

 

my gut instinct, therefore, is that the shaken can will go faster. I would be surprised if the difference is large.

[han]

Come on people, this is important. Let's get to a consensus.

[kfay]

They travel at the same speed. Pressure in the two cans is equivalent since the cans are under pressure to begin with. The reason why pop sprays when you open it is that the soda now is not under pressure but the molecules have more energy allowing the gas to escape more readily. But prior to that there is no change in the can as far as the outside world is concerned, it's a 'closed system.'

 

So for the purposes of rolling down hills, the cans are equally speedy.

[inquiry]

amazing, it seems from a trial here that the shaken can is always slower by experimental evidence. Why this happens I can't say. I will go with the can expands a bit, but it doesn't really look to be the case to me....

 

word of caution, open the shaken can VERY carefully.... :)

[helene_t]

If one can has higher viscosity than the other, it will accelerate more slowly, as it does not allow the time lag of rotational speed of the cola relative to the can. Try to fill one can with honey and the other with alcohol, you will notice the alcohol can runs faster.

[gwnn]

the question is: is liquid-liquid friction higher than liquid-gas friction? the answer is intuitively negative, but I'm not sure.

[Fluffy]

intuitively I'd say that the higher the presure, the higher the viscosity, for the same reason that I am a lot less agile if I have table over my shoulders. Can be a total nonsense :).

 

 

 

Another thing is what happens with the temperature, if shaken cola has higher temperature, I think it has less weight, and then goes slower.

[helene_t]

No, the weight is unchanged. But the higher the temperature the lower the viscosity.

[matmat]

No, the weight is unchanged. But the higher the temperature the lower the viscosity.

unless there is some sort of transition there, i doubt the temperature rises enough to change the viscosity significantly.

 

(edited)

[han]

The shaken can is clearly slower. Scientists don't agree yet about why this is the case but it is very clear if you check it experimentally. Well done Ben and others.

[Fluffy]

This reminds me of why sport teams do better when playing at home rather than trveling away, there are loads of possible reason, but the real answer is the sum of all of them.

[Al_U_Card]

My answer (better late than never) would be that liquid to solid (can) friction makes the difference. The shaken can releases CO2 by nucleation, temporarily increasing the gas pressure in the void space of the can. (until the CO2 is reabsorbed by the liquid, more quickly at lower temperatures)

 

This increase in pressure is transmitted by the incompressible liquid to its contact point with the inside of the can. Friction increases with pressure so therefore there is more drag on the "rolling" can so that this slows its rotational speed. (Like a "drum" brake applying more pressure to the wheel makes it slow down more.)

[Penguin]

I asked a physics professor friend of mine, a non-bridge player this very important question, and he replied:

 

*********************************

 

Wow, and I thought all bridge players thought about was sex.

 

OK, there were a couple of theories put forth, and I can cross a few of them off the list.

 

1) Viscosity. Yes, a can of honey would roll more slowly than a can of water because honey is more viscous. The difference in viscosity between shaken and unshaken soda, however, is negligible.

 

2) Sloshing. To a good approximation, as the can rolls down the incline, the liquid will be at the bottom of the can in a relatively tranquil state. Any "sloshing" would happen from irregular movement, not from smooth acceleration down a flat incline. Any argument involving the turbulent movement of the liquid therefore seems irrelevant to me.

 

Here's what I think: The released gas causes increased pressure inside the can, causing the side to bulge out just a tiny bit further. This increased radius at the midsection causes an increase in the can's "moment of inertia." Even though the mass of the can doesn't change, this increased moment of inertia impedes rotational motion, and the can therefore rolls more slowly.

 

Someone guessed that the expansion of the can leads to more external air resistance acting on it (because the larger an object, the more it is slowed down by drag forces). I'm pretty sure that the increase in size would be too small for air resistance to be a factor.

 

Comprende?

[jtfanclub]

2) Sloshing. To a good approximation, as the can rolls down the incline, the liquid will be at the bottom of the can in a relatively tranquil state. Any "sloshing" would happen from irregular movement, not from smooth acceleration down a flat incline. Any argument involving the turbulent movement of the liquid therefore seems irrelevant to me.

I think that's a myth.

 

There is considerable surface tension between the cola and the can (which is why if you get coke on the outside of the can it gets all slimy). If you think about a partly full can, that coke on the outside is going to follow the can to the top and fall down to the bottom, creating considerable sloshing.

 

If that's true, then a half full can of coke will take just as long to slide down as a sloshed can (which has a lot more free gas in it) in contrast to a full unsloshed can.

 

Somebody will have to test this....

[helene_t]

Here's what I think: The released gas causes increased pressure inside the can, causing the side to bulge out just a tiny bit further. This increased radius at the midsection causes an increase in the can's "moment of inertia." Even though the mass of the can doesn't change, this increased moment of inertia impedes rotational motion, and the can therefore rolls more slowly.

This was my first thought as well but I thought I came to the conclusion that the moment of inertia has no effect on the acceleration of the can. I may have been thinking wrongly.

 

The rotational energy is 1/2 I omega^2 and the translatoric energy 1/2 m omega^2 r^2, For a homogenous cylinder I is proportional to m r^2 so it seems to me that the rotational-to-translatoric energy ration is unaffected, and that this is the magnitude that determines the can's inertia. There could be something wrong with my reasoning or the clue could be that the can is not a homogenous cylinder.

[matmat]

Objects with a higher moment of inertia take longer to get rolling. same way that heavier objects take longer to get moving with pure translational motion. (under the action of the same force).

 

I've already been wrong about this once...

 

does anyone have a couple of clear bottles of soda to perform this experiment with? :P