stuntSo I am rehearsing a new (and as it turns out, somewhat dangerous) stunt for an unusual talk that I have to give next month. All you need is a big stick, two wineglasses, and an even bigger stick…

I am rehearsing a couple of science-based demos for a talk next month, and came across a great-sounding piece in which you balance a big stick between two wineglasses, and then hit it with an even bigger stick. According to the laws of physics, the stick breaks and the glasses remain intact. When developing pieces for Theatre of Science, we always had two simple rules – safety first and make sure it works every time. As you can see from the rehearsal footage, all is going swimmingly….

Anyone have any ideas why it doesn’t work?


  1. Thanks for the out-take, it made me smile.

    The mass of the target stick was too great. One that is of smaller diameter/lower density but still capable of breaking a glass when used alone, is the answer, methinks.


    Richard (Microbiologist – never got beyond A-level physics, and that dates me)

  2. Heh Bugs Man, there are really people out there who believe the MR stuff!!! IMHO, I would suggest that mass is less relevant compared to tensile flexion… the energy travels along the length of the stick as it bends and this transmits to the more rigid glass.

  3. Actually,, I am not sure about ‘tensile flexion’ .. I think I just made that up… a bit like morphic resonance.

    I agree the stick is not brittle enough!

  4. I hadn’t finished! Silly laptop.. Because you didn’t hit it dead centre..AND because the stick you hit it with was too thick in comparison to the one betwixt the glasses!

    Your reaction made me laugh.

  5. I’m also wondering of the angle of the strike had anything to do with it. Does the stick coming down have to be parallel to the floor when it hits the second stick? I’m thinking that would be the most structurally sound. Then again, I’m no physicist. 🙂

  6. Hi Richard,

    Off topic, but I thought that you should know that when Google generated its feed from your blog for my Google Reader, it calls your blog title “Undefined”. I don’t know where the glitch originates, but it’s not a problem I’ve had with any other feed.

  7. Try using a bamboo stick balanced on the wine glasses and try balancing it on both edges of the mouth of the glasses. It should be strong enough to withstand the downward force of the strike then.

  8. Yeah, what Sid said.
    The glass broke because the stick was balanced only on one side of the glass, the force was not transferred vertically through the steam. The upward force from the table tilt the glass anti-clockwise.

  9. Reminds me of the old rock-on-the-chest stunt the martial artists and fakirs perform.

    Striking on the centre and making sure your force is directly downward at the point of impact are both clearly important but you seem to be on top of that. Does your theory suggest how accurate you need to be?

    I’d agree with other posters that changing the stick itself is your next step.

    You’ve already got the stick pretty long, but make sure it’s as long as you can. Make its own weight work against it. But basically choose a wood which is weaker – more brittle, with less spring in it.

    Hope that’s not too much of a cop-out.

  10. I assume if the trick works, it is because of the inertia of the stick-between-the-glasses. The force of the hitting-stick is supposed to be fully absorbed by the stick-between-the-glasses and not transmitted to the glasses? So, a bigger stick-between-the-glasses is, perhaps, called for.

  11. I don’t think it’s to do with the strength of the glasses – in an Ideal Situation (TM) the glasses wouldn’t experience any extra force at all…

    (p.s. this is very long and shows just how many interesting physics factors have occurred to me while writing this!)

    Imagine shooting it with a gun, would you expect the entire mass of the stick to be forced downwards, breaking the glasses? Or would you expect the pieces to end up spinning about their centres of mass, like the bones in the dawn scene from 2001? (I know the forces are entirely different, but the mental picture is the same).

    How about this –
    Imagine a stick floating in space (!)
    It is struck in the middle by a weapon with non-zero width. Of course if you had a weapon of zero width then you would not be breaking so much as cutting, which would also be pretty cool, but then I’m sure you would be doing much more interesting demos than this…
    The stick quickly breaks into two pieces and – with the weapon having non-zero width and a bit of follow-through – the weapon continues to apply force to the broken ends of the new pieces (ignore bending for now – clearly it makes it more complicated but it shouldn’t ruin the principle)

    Consider two periods B.C (before cracking), and A.D. (after division)

    B.C. – the force is applied to the stick as a whole, so the whole stick accelerates in the direction of the applied force (if bending is ignored – a “perfectly brittle” stick – then B.C. will be infinitely short …). This would equate to a downwards “glasses-breaking” force in the real experiment

    A.D. – the continuing force is applied to the broken ends of the two pieces. They will begin to increase rotation (about their centres of mass, naturally) because of the off-centre force. Their centres of mass will also continue to accelerate in the direction of the applied force (though at a lesser rate).

    (if you don’t believe me, try hanging a horizontal stick from its middle by a long piece of thread, and then flick one end horizontally away from you – the other end will come towards you – nothing to do with gravity)

    So, the instant after the break, the centres of mass of the pieces will be traveling in the direction of the applied force, but if B.C. is short enough relative to A.D. (i.e. there is little enough initial simple acceleration) the *far* ends of the pieces of the stick will be traveling “backwards”, because of the rotation of those pieces. This would equate to “upwards ” in the real experiment.

    Put the stick in the real world and add gravity and there will indeed be some force down on the glasses, but make the A.D.:B.C. ratio sufficiently high:low (and make the rotating force suficiently high), and the outer ends of the stick could even jump up off the glasses while the middle is forced downwards.

    Now how to make it work and look good. You could use a breadstick but that wouldn’t be terribly impressive – this demo is not about efficiently breaking a stick (if you want to do it efficiently, use a sword), it’s about breaking a sturdy-looking stick with as much fury but apparent delicacy as possible.

    The target shouldn’t necessarily have to be light – the ends will still travel backwards if the impulse is great enough.
    Due to the inertia/bendiness of the stick (and all real-world objects) you can imagine the force of the blow traveling in real time along the stick to the ends – like a shock wave. You want the stick to break before the shock wave reaches the glasses. It’s not a matter of getting the stick as light and brittle as possible (“a breadstick”), but getting one brittle enough so that it will break before the shockwave reaches the ends, but flexible enough to absorb a lot of energy from the weapon before it breaks so that the ends spin round a lot. That would also imply to me that you want the weapon to be good and heavy so that it is not slowed down by the impact and can continue to apply force to the broken ends after they have broken. Of course, that will make it harder to get it to a good speed!

    So I reckon it’s not a case of making anything as brittle/light/long/flexible/strong/heavy as possible, but a balancing act of various factors. Impedence matching, something like that.

    Get as much energy into the weapon as possible. It’s difficult to get energy into something light using real mechanisms (like arms), even though it might end up moving more quickly – I believe an arrow will go straight through a piece of bullet-proof glass. You can get more energy into a sword than a pen (though the pen may be mightier, the sword is heftier). However, it’s difficult for something moving slowly to impart its energy in an impact with something light (impedence matching again) – a metal pipe would probably be a good balance between strength, weight, hardness and heftability.

    Good broom sticks are designed to be strong and bend rather than break. Cheap sticks are more likely to break for the same force. I suspect a lighter stick will be better, not so much because of its lower weight, but because of its lower strength. Get a crappy one from a pound shop which is little more than a piece of dowel, rather than a proper centre-cut turned broomstick which will be much stronger (because of grain density, or something…).

    And hit it as hard as you can. The harder you hit it, the less force you’ll give to the glasses!
    I think.
    In an Ideal Situation (TM).

    I appear to have written a short dissertation. I’m sorry, I have a cold and my brain is running. Hopefully it makes sense and gives people something to think about. There are lots of gaps – but that’s part of the fun, don’t you think?

    Share and Enjoy.

    p.p.s. And I don’t think it necessarily has to be hit in the middle, or directly downwards 🙂 but it is optimal.

    p.p.p.s. I’d be surprised if bamboo worked better – bamboo is usually wonderfully bendy.

    p.p.p.p.s. And I suggest practising with plastic cups….

  12. I think fizzybrain’s got the theory spot on. As well as getting the target snapped before the shockwave reaches the glass, you don’t want to still be applying force when the stick has bent and is only touching one side of each glass’s rim. I reckon the tool for the job is a sledgehammer – it’s designed to deliver maximum energy in an instant so long as you give it a good swing – and it would look great! Your aim would need to be pretty good though. 2×1 might make a good target – easier to get balanced on the glasses. It’s just as well it’s dark outside or I’d be experimenting myself!

  13. I think Chris is about right. My guess is the target stick should be less flexible so that it breaks and bends almost simultaneously, thereby putting less stress on one edge of the glass.

  14. Your (only) problem was/is that the stick you were striking with bounced off the target stick…

    You want to be able to hit THROUGH the stick… i.e. pick a spot BEYOND the big stick as your target and hit that… perhaps have a chair/stool set up with a cushion on it to act as something for you to focus on (and to prevent you from striking the floor with excessive force and sending shockwaves back up and through your arms!) whilst you’re working out just how much force is required.
    Also, it will help to consider the physical properties of both your target stick and striking stick in order to be able to hit through…. You will require for both wood of as little flexability as possible. You’re striking stick should also be of equal or greater strength to your target stick [yes, it CAN be of equal strength as the force applied by it in the collision will first contribute to accelerating the target stick in the direction of your strike (thus simultaneously causing the two halves of the target stick either side of the impact point to pivot away from the glasses) before providing the reactive (normal) force which would be the first shockwave… which could potentially shatter both or (as in your case) neither of the sticks].

    Solve this problem and everything else mentioned above will solve itself (or vice-versa).

  15. Richard,

    Thanks for the link to my high-speed video! I just noticed that it started going viral (more than my usual videos, that is) and tracked that back here to you.

    A couple of comments about your setup.
    1. Make sure that you’re using a nice dry pine board. I tried it with poplar and that turned out to be “stretchy” and didn’t snap quickly like pine does. Also stay away from oak or anything stronger like that. And be sure that you’ve got a nice clear board without any knots and free from any “interesting” grain patterns. It might not break where you want it to.
    2. Try using a heavy metal rod. Or a long section of rebar. My video is done with a 1″ steel bar.
    3. Make sure that you hit it with as much speed as possible. The force of the impact is a product of the velocity and the contact time. So, doubling the speed actually gives you 4 times the force. I note that you’re backed into a wall on your backswing. Give yourself a little more space. You did well at hitting the stick straight and centered. But you gotta hit it very fast!
    4. I can’t tell if you’re wearing safety goggles. I do that ever since I was hit in the temple by a broken shard of wood.
    5. I’ve broken 3 wineglasses doing this myself. It always amazes me that you can understand the physics, have seen it done, and have even done it before but can still clutch when you get up there. Something in the brain looks at the setup and says, “Hold on there! That’s just not going to work!”

    Good Luck!

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