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I don't know anything about physics so this may sound dumb. I punctured a hole on the top of a condensed milk can to drizzle on my snow cone, but when I tried to poor it it barely came out. I randomly had the though to poke a hole on top still, but opposite side to see if the air would help it flow. It helped a ton! What is the science behind this? I'm assuming it has something to do with suction.

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atmospheric (air) pressure, or barometric pressure will hold the liquid in the can, a second hole, depending on it's location will equalize the barometric pressure allowing the liquid to flow freely.

there is a simple experiment with a partial glass of water and a piece of cardboard (think cereal box), where the barometric pressure will hold the cardboard and water in an inverted glass.

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If you only have one hole, liquid will flow out slower because air will need to replace the liquid inside the can, and the same hole the liquid flows out of is where air will go in. If you have two holes, the milk can continuously flow out one whole while air continuously goes into the other hole.

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Did you ever have to punch holes in cans when you were young? We had to do this with big cans of tomato juice - put two holes in it on opposite sides.

One is for the juice to come out, and the other for air to go in.

When you pour out a coke, the can doesn't just become a can of nothingness - it gets replaced by air. The hole in a coke can is big enough that it lets coke out in a small stream and air can get in at the same time to replace it.

However, in the case of your tiny hole in the condensed milk can, it wasn't big enough for the replacement air to get around the milk which needed to come out.

So when you poked another hole, air could get in so your milk could get out. Voila! You could have also solved this problem by making the original hole bigger - but it's easier to just make two small holes on opposite sides.

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