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Coke and Mentos Powered Car Answered

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



33 Replies

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Doctor What (author)2010-06-03

 I see a lot of energy wasted here...  They could have propelled themselves much further.

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kelseymh (author)Doctor What2010-06-03

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.

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fungus amungus (author)kelseymh2010-06-07

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.

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Kiteman (author)kelseymh2010-06-03

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:


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Jayefuu (author)Kiteman2010-06-03

Awesome video :p don't show it to any of the kids at your school

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Kiteman (author)Jayefuu2010-06-03

Too late - that was broadcast by the BBC at prime time.

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Plasmana (author)2010-06-07

I have to say, it is really cool, but really inefficient...!

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Plasmana (author)Jayefuu2010-06-07

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.

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Minifig666 (author)2010-06-06

If only they could impliment this with a V8 engine. :D

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Kiteman (author)2010-06-02

At last, a use for Coke Zero!

(What, you don't mean people drink that stuff??)


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Goodhart (author)Kiteman2010-06-02
Admittedly,  the Zero name means zero taste as well as everything else :-)
 

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Kiteman (author)Goodhart2010-06-02

I find it sickly sweet.

(Whereas Kitewife finds sugary coke to be too sweet.)

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Goodhart (author)Kiteman2010-06-02
Really ?   I wonder if the formula is different for those sent across the pond?  The stuff here has nearly Zero taste (like seltzer water, really).   Tonic water has more taste, IMHO
 

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Jayefuu (author)Goodhart2010-06-02

It tastes like more chemical dosed coke.

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Goodhart (author)Jayefuu2010-06-02

It must be different over there.   It isn't my taste buds,  I can still pick out an assortment of herbs and spices in many types of food stuffs; and the coke zero I've had is quite without flavor....or I got a hold of an old batch maybe....

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kelseymh (author)Goodhart2010-06-02

Okay, this has gone on long enough.  Somebody needs to step in....

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Minifig666 (author)kelseymh2010-06-06

It's like an advert for the base 10 countig system! I can't comment about coke zero but coke seems to taste diffrent depending on how it's bottled. Glass normaly tastes better. Even it the UK the coke tastes slightly diffrent depending where you are.

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Goodhart (author)kelseymh2010-06-02

NOT to be confused with Zorro.....

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Chewie42 (author)Goodhart2010-06-04

It is actually good in the states..we dont let you limeys have the good stuff :-)

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Goodhart (author)Chewie422010-06-05

"good" is a relative term. It doesn't taste like much of anything to me (here in the states) ;-)

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jeff-o (author)Goodhart2010-06-03

It actually is different between regions.  For example, regular Coke in North America is made with corn syrup, where in south america (and mexico, I think?) it's made with real sugar cane.  It tastes much better down south, AFAIK.

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caarntedd (author)Goodhart2010-06-02

Tastes OK down here. But then I have put a couple of shots of rum in it.

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KentsOkay (author)Kiteman2010-06-04

A bloke at work drinks it a lot.

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Kryptonite (author)Kiteman2010-06-04

No. It's a lie. They don't. I work at McDonald's (first job :D ) and when ever someone orders Coke Zero, I watch them closely only to see them throw it on the floor. And that's how cleaners get their job.

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Spycrab (author)Kiteman2010-06-03

I find coke zero delicious :D
tastes like coke, for me.

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ANDY! (author)2010-06-05
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corey_caffeine (author)2010-06-05

nope, no waste of resources here, no sir...

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V-Man737 (author)2010-06-05

The music! The warehouse! The scientific application! These guys make being a nerd cool !!

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fungus amungus (author)2010-06-03

Dude, give the guys credit. This is EepyBird and they've been making cool videos for a while now.

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Jayefuu (author)fungus amungus2010-06-03

Woops! I saw your email then forgot. Added :)

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M.C. Langer (author)2010-06-03
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kelseymh (author)2010-06-02

Now that's what a Mythbusters segment ought to look like (and what they used to do....)!  Actually showing the fabrication and assembly, not just big explosions.  Very nice, and highly efficient design, too.

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