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Coke and Mentos Powered Car

Eepybird, the "Coke and Mentos Guys" used 108 2L bottles of Coke Zero and 648 Mentos to propel a car/bike 221 feet!

An explanation of how it works, from their YouTube video:

"The Coke Zero & Mentos Rocket Car uses a piston mechanism: a six-foot long rod sits inside a six-foot long tube attached to each bottle of Coke Zero. When the Mentos drop into the soda, the pressure tries to push the rod out of the tube. With 108 rods all pushing at once, that gives us a lot of power.

All that power is pushing against a wall braced with 3,600 pounds of cement blocks. So all the force is directed into moving the Coke Zero & Mentos Rocket Car forward. We get one big push for six feet, and then it's all coasting from there."
 
via msnbc




Picture of Coke and Mentos Powered Car
mentos-car.jpg
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Doctor What7 years ago
 I see a lot of energy wasted here...  They could have propelled themselves much further.
That's not obvious to me.  The jet output is relatively low velocity:  a single Mentos only gets you about a 3-4 foot vertical fountain, which means a maximum velocity of just 3.3 m/s, in 0.3 seconds (i.e., 1g). 

The thrust is not linear with the number of Mentos, so we should assume the acceleration isn't much more than that.  If the whole 2 liters gets used, then you've got 2 kg at 1g, so 2 newtons of force per bottle. 

They used 108 bottles, which gets you 216 newtons to get the bike going.  Since the mass of the bike is more than 216 kg (trivially, since it's all the Coke plus the support structure, plus the bike and rider), you're not going to get 1g acceleration of the system, but probably more like .5 g or less.

Now, that's all just back-of-the-envelope.  For a homework problem, work out the numbers in more detail.
Looks like there were a couple of places they could've improved things.

They're dropping the mentos into the bottles by removing pins that are holding them up. The fine spray is coming out of the holes the pins go through. A magnetic system would be better.

The seal of the pistons was pretty loose and allowed for a loss of pressure before the brake was released.

It's difficult to tell just how much of an effect these had due to the editing which looks like it had the effect of stretching out the time to show everything happening, but those would be good places to look at.

Of course it's also underwhelming to watch the whole thing slowly coast to a stop which is pretty anticlimactic. Nothing they could do about that.

I think he's referring to the sideways-shooting coke.

If the launch tubes and rods had been slightly longer, they would have used every last scrap of energy.

See:


Jayefuu (author)  Kiteman7 years ago
Awesome video :p don't show it to any of the kids at your school
Kiteman Jayefuu7 years ago
Too late - that was broadcast by the BBC at prime time.
Plasmana7 years ago
I have to say, it is really cool, but really inefficient...!
Jayefuu (author)  Plasmana7 years ago
Why so?
If you look at it, tons of coke and mentos used and they did not go so far as I expect them to go, like I said, it is amazing though.
Minifig6667 years ago
If only they could impliment this with a V8 engine. :D
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