Episode 163: Bubble Trouble

Air Date: April 27, 2011

It is impossible to swim in bubbling water.

plausible

In a small-scale test of buoyancy in bubbling water, Adam built a crude hydrometer, weighted to float at a certain height, and placed it in a fishtank full of water. The device did not sink when air bubbled in, but he and Jamie thought that this was the result of upward water currents. Jamie then built a larger bubbler to place inside a 10,000-gallon tank (previously used in the whirlpool myths). Adam donned a wetsuit and carried enough weights to leave only his head above the surface. When the bubbler was turned on, the upwelling pushed him to one side, where he sank in a downward current.

In order to eliminate these wall effects, Adam and Jamie built a 4-by-16-foot bubbler to place at the bottom of a swimming pool. After they added weights to keep the rig from floating up, Adam tried to swim across the pool and back through the bubbles. The trip proved difficult at 25% power and impossible at 100%. Adam and Jamie classified the myth as plausible, but for a different reason from the one expected — water currents holding the swimmer at the surface, rather than a loss of buoyancy due to the air bubbles.

If a stick of dynamite is attached to an arrow and shot into a tree, it will split the tree down the middle when it explodes.

busted

To simulate a real tree, the Build Team dug a hole in the ground and stood a 20-foot, 6,000-pound (pine log in it. They set up a remote-controlled rig to fire an arrow fitted with a binary explosive charge equivalent to a stick of dynamite. Tests with a single and double charge failed to damage the log, so they stuck an arrow into the wood by hand and attached six charges. This attempt also did not result in a split; Grant commented that the placement of the explosive outside the tree surface prevented its force from being channeled into the wood.

Declaring the myth busted, the team did some small-scale tests with different explosives and placement techniques. A TNT charge drilled into the trunk shredded it at the blast point, while an ANFO charge laid in grooves cut along the wood grain caused some degree of splitting.

In one last full-scale test, the team chose a 100-foot Ponderosa pine and fitted it with 25 pounds of ANFO, laid in grooves cut near the core. The resulting blast tore the tree into hundreds of pieces.

16 Comments

  1. Arthur says:

    When splitting the tree, why not use what the pros use to bring down a building. They used shaped charges or blades backed with explosives that do the cutting. This would be closer to splitting a log with an axe or a mallet and wedge.

    • T. Rollins says:

      The myth was: “If a stick of dynamite is attached to an arrow and shot into a tree, it will split the tree down the middle when it explodes.”

      Not: “If a shaped or loaded charge….”

  2. austin dean says:

    would you try to see if a mower and car come together and will the mower turn in to a ramp or explode

  3. Bryan says:

    As Arthur states, a shape charge would probably spit a tree. I would also add running the length of the tree with High-yield detonating cord. I know it can cut very adequately through concrete block and adobe walls.

  4. stu says:

    I feel like the bubble test needs a re-do. I’ve seen my city’s aeration systems used to treat sewage and was told that due to the low density of the sewage meant if you fell in, you’d quickly sink the 40+ feet to the bottom, pretty horrible. Mind, these are fully aerated columns without surrounding stagnant water. Add to that the organic material in the water, and it is quite different. I’d like to see this one redone where there is no non-aerated area. Plus my tv guide thing erroneously summarized the myth as “would you sink in carbonated water”. You won’t, but it sounds like fun, and I’d like to see it.

  5. Larry Robinson says:

    I would also like to see the bubble test redone. I believe it was on one of the TLC shows not a Mythbuster’s show. It was more a show about the Bermuda Triangle than not being able to swim in air bubbles in the water. They were trying to prove that gas escaping for under the crust or bottom of the floor was causing large ships to sink. They had laid their air hoses about 30 feet down on the bottom. Yes they had to adjust how much air was being released to get the right density and buoyancy to work.

    • J Smith says:

      Larry, did the test work i.e. did the ship start to lose buoyancy? Did you mean lack of boyancy in the last sentence?

      I agree a more thorough re-do is in order… the whole pool needed to be bubbling.

      JSmith

  6. skip says:

    love mythbusters but blowing up a living tree is really “jumping the shark”

    • Anna says:

      I agree, to me that’s like killing a living thing- (although I am a tree hugger).

  7. Vojtech Slama says:

    Since Mythbusters e163 I hate this show. Blowing up so many trees was awful! Next time I will watch more sensible shows.

  8. Jacques says:

    Thought experiment – if the air was held in place, say using a number of balloons filled with air, tied with strings to the bottom of the pool, the “density” of the water would still be considered lower by that logic, but no one would think it would cause you to sink. It makes no difference that the air is split into smaller pockets when using bubbles – only when the air is dissolved in the water could it change the water density in terms of how it affects a body trying to float in it. (Before you factor in the upwards currant.) You mistook the density of the system measured from outside with the density of water relative to an object in the water when there are other “objects” in the water with it. It should at least have been addressed.

  9. J Dyer says:

    Bubbles of different sizes must cause a different reaction to a floating item.

    Years ago we placed rubber balls ( with specific gravity of about 0.9 ) in a vat of water which had air diffusers in the bottom. This was a trade-show prop to expose that the diffused bubbles would change the specific gravity of the water around the ball to allow it to drop when the diffusers were on. For 3 days, and 20,000 people, and numerous on and off cycles the various colored balls quickly sunk when the air was on, and, bobbed to the surface when the air was off.

    On the show, I was surprised to see the small boat, and Adam, obviously rise out of the water when the bubbles came up….especially after years of me telling industrial clients that bubbles make floating things sink.
    Guess there are just lots of occasions when scientific truths are sometimes true…..
    Dyer

  10. viv says:

    Im not sure if the myth was amped up to ‘an arrow’ but using explosives to split trees really was a valid technique. A modern day reconstruction was performed in the UK when ‘Grand Designs’Kevin McLeod did this to create the timber for his man made home check channel4 programmes

  11. Ben says:

    Ironic in this show that Bear Grylls was mentioned (but in relation to splitting the tree). But it was BG’s words in my mind around the aerated water experiment – he was talking about not jumping into such water at a waterfall so you would not sink. Perhaps a re-test based on dropping Buster. Also support Larry’s suggestion where the whole test pool is aerated to minimise effect of the observed ‘convection’.

  12. Lars says:

    I have split a few huge logs and a birch tree on root with explosives in the boy scouts, more than 30 years ago.

    In Mythbusters they used way too much and too fast explosives.

    We used drilled holes, very small amounts of black powder (maybe 5-10 grams), electric ignition and sealed it with a wooden plug. Worked every time. The logs had two charges, a bit apart and on opposite sides. The tree only had one charge, and split from the root almost to the top (it was used as firewood later) .

  13. Tamfang says:

    On a tangent: when Kari is wiring the tree at the end, it appears to me that something around or behind the cord was hidden (background pasted in) in post-production. ??

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