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More specifically: could a 2-litre milk carton like the one pictured below be used to knock someone unconscious via a single blow to the head, or would the carton burst from the force?

Here is some information I could find on the carton and the milk:

The rectangular base is approximately 9.5cm x 9.5cm, and the rectangular prism portion of the carton is 21cm tall. The total height of the carton is approximately 25.5cm tall. The milk fills the entire rectangular prism but not the rest of the carton. The total volume of milk is approximately 9.5cm x 9.5cm x 21cm or 1895.25cm-cubed. That gives an approximate volume of 1.89525 litres of milk and an approximate mass of 1.9521075 kg of milk (the density of milk is approximately 1.03 kilograms per litre).

I've thought about asking this on physics.SE or biology.SE but I don't know if it would be considered on-topic.

If this question would be better suited somewhere else, please migrate it.

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    $\begingroup$ As written this just doesn't sound like a Worldbuilding question. If you could supply a context and goal for your story/world it might be retrievable. You might like to read How to safely knock someone out and answers therein. $\endgroup$
    – StephenG - Help Ukraine
    Commented Mar 20, 2018 at 1:06
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    $\begingroup$ It's not a matter of 'or'. If propelled with sufficient force, it could knock a person unconscious while bursting in the process. Most of the energy/momentum is contained in the liquid milk: if you think in slow motion, you'll see that the force will first be applied to the person's head (good old Newton's Law. It's the rebound effect - like a splash - that bursts the carton. $\endgroup$
    – jamesqf
    Commented Mar 20, 2018 at 5:03

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A lovely quote comes from the movie Valkyrie:

Any problem on Earth can be solved with the careful application of high explosives.

You can knock someone out with a stream of water if the water is flowing with enough pressure. Therefore, given enough velocity, a 2L carton of milk will rip a person's head off. Your real problems are these:

  • Can I effectively accelerate a 2L carton of milk to a velocity that would render some poor schmuck unconcious (because the only thing more embarrasing than being knocked out by a carton of milk would be getting knocked out with a Haddock)?

  • Can I aim the carton well?

  • Will I hit the right spot? There are different ways to knock someone out. With a carton of milk, hitting the forehead is likely better than hitting the jaw, the temple, or behind the ear. You want all the leverage you can get.

  • Truth be told, there is no magic number (force in joules) that will knock out every human, or even an average human. There are too many variables: everything from their height and weight to whether or not their neck muscles have been strengthened. Separation between brain and skull, viscocity of cranial fluid, whether or not the wind is against you....

  • There's also the burst strength of the carton. Granted, if you accelerate the carton fast enough, even the burst strength won't matter ... but if someone is throwing the carton, it matters a lot. The carton would need to be unopened and having a burst strength that would keep it from breaking open if you lobbed it, say, 10 feet away from you and let it hit the ground. These are ROUGH AND NEARLY USELESS numbers, but the fact is, the easier it is to break the carton, the harder it is to knock someone out.

So, can you do it? You bet. Is it believable? Only if whomever is throwing the carton is literally giving it everything they've got. And even that might not be enough.

Therefore, I'm going to give this one a 75% smirk factor — which is the measure of how likely someone is to smirk if they read about this in a story. Definitely higher than normal ... but not impossible. (Knocking someone out with a feather duster would be a slammed 100% smirk factor, just for comparison.)

By the way, the folks over at Physics.SE and Biology.SE are purists (and rightly so), which is a fancy way of saying they wouldn't answer a question like this because it's not serious enough. I asked them how long the copper and zinc rails inside the barrel of a potato gun would need to be to power a sound effects circuit — I had some comments, but no answers.

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    $\begingroup$ Physics SE can't answer this because "knocking someone out" is not a well enough defined action. Biology SE probably can't for the same reason. $\endgroup$
    – StephenG - Help Ukraine
    Commented Mar 20, 2018 at 1:01
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    $\begingroup$ @StephenG, you are, without doubt, correct. They also don't like answering creative questions. That's OK, discussing their sciences is what they were created for. My point is that they do their thing, and this question doesn't quite fit in their thing. $\endgroup$
    – JBH
    Commented Mar 20, 2018 at 1:42
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    $\begingroup$ @JBH - why do you assume that the milk carton would be thrown at the victim's head? Isn't it possible that the milk carton would be used like a club or a blackjack and smashed against the victim's head? $\endgroup$
    – M. A. Golding
    Commented Mar 20, 2018 at 17:04
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    $\begingroup$ @M.A.Golding, I took the OP at his word that he wanted the milk carton to be the impact object that knocked someone out. Holding the carton in your hand means you've lost the leverage of throwing the carton. At that point, it's your hand that's knocking them out. The carton, unlike brass knuckles, isn't hard enough to compensate for the lack of swing with a non-elastic collision. The force needed for the carton, itself, to be responsible is very high. I'm not sure a human arm could do it (baseballs are easier to hold than cartons). Therefore, no, I don't think it could be a club/blackjack. $\endgroup$
    – JBH
    Commented Mar 20, 2018 at 17:13
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I think the most reasonable answer is "it depends." There are a lot of different variables in play in striking someone with an object. If it were only a matter of mass, the answer would be yes - a fastball weighs much less and can cause unconsciousness. Is the milk in the carton frozen? A 2kg weight like that could be a deadly weapon.

If the milk remains liquid, then it seems as though it's not going to transfer all of its kinetic energy to the back of the victim's head, tending to burst the carton instead. Someone smarter than I can figure out what the bursting point of a milk carton is, and what percentage of the potential energy is lost to spray out.

Is the carton being swung in someone's hand? A punch can knock someone out, and your hand doesn't weigh 2 kg. Is the carton falling off a shelf, or off the top floor of a building, or being thrown from a moving car? All of those impart different speeds to the object, so each scenario would change the odds of a single-blow knockout.

I'm led to believe that the thing that actually makes you unconscious is your brain bashing against the inside of your skull. Therefore, it's not the weight of the object per se, but how rapidly that object can make your head move upon impact. If the carton can hit your victim just right, nighty-night. If not, you just get someone pissed off and covered with milk.

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    $\begingroup$ Of course, one can add a whole host of possibilities stemming from the milk carton even just being enough to stagger someone - if they hit their head off something else because they're off balance, all the possible things and ways they hit their head could cause even more variables $\endgroup$
    – Megha
    Commented Mar 21, 2018 at 6:01
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    $\begingroup$ And, of course, that "someone" could be a newborn. $\endgroup$
    – RonJohn
    Commented Mar 21, 2018 at 17:37

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