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When I stir my coffee in a cup, I see the foam on top of it rotating (notably, in the center of the cup). Now, if I rotate the cup alone, nothing happens to the coffee and it continues spinning as I left it. However, if I start myself spinning around with the coffee cup in my hand, the coffee slows down and comes to a halt at some point when I spin around fast enough. I presume that if I spun the opposite to stirring direction, I'd make it stir more, although I didn't perform the experiment as I have already drunk the coffee. Work is waiting.

In a similar fashion, if the coffee has not been stirred, and I rotate the cup alone, nothing happens. But if I start spinning clockwise, the coffee starts rotating counterclockwise in my hand. This is puzzling: it does not apply to solids, as I don't feel that my body wants to rotate around its axis when I take a sharp turn in the car.

First, why are those two situations (where the axis of rotation goes through the center of the cup vs outside of the cup) different?

Second, why, if I start with unstirred coffee, and start spinning around, don't I see the coffee just receding to the opposite side of the cup without rotation? Would that happen with a more viscous liquid?

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    $\begingroup$ No way! I found another person who does mad experiments on their coffee in the morning too. I feel better about myself now... $\endgroup$
    – Tachyon
    Commented Jun 15, 2021 at 23:52
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    $\begingroup$ @Tachyon We can form a trio... :) The spinners $\endgroup$
    – user304539
    Commented Jun 16, 2021 at 0:02
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    $\begingroup$ @Viesik We can make a club and make a podcast for this. It will be great! ;) $\endgroup$
    – Tachyon
    Commented Jun 16, 2021 at 0:06
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    $\begingroup$ @Tachyon There is a lot of weird stuff to chose from! My wife is calling me... I sit too long behind the screen, she says. Tomorrow I'm here again> $\endgroup$
    – user304539
    Commented Jun 16, 2021 at 0:10

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I like the way you tell the story! Especially that you had no coffee left because you drank it all! The coffee inside your cup has the tendency to stay as it is considering the state of motion (law of inertia). The coffee is not tied to the cup strongly so when you make a spin it will stay where it is and seen from you it will spin in the opposite direction. I see the coffee standing still (unless I spin along).
If you are in your car and make a turn, your body initially wants to stay where it is too. But because it's a solid whole (more or less) all parts make a turn too so it doesn't rotate (maybe the water in your bladder makes a counter turn as seen from your turning frame). You will feel a force though that's pulling you to the side of the car (and one that wants you to rotate, but you don't feel that because it's a small force). Likewise, the coffee as seen from our rotating frame will crawl up the coffee cup (away from you) and it makes the coffee rotate if you start to spin, after which the rotation speed doesn't increase. The coffee, when spinning at a constant speed, will rotate at a constant speed, But due to friction, the spinning will slow down. But seen from my rest frame the coffee will start to rotate.
Hope this helped! Try again when you have new coffee and don't drink it too soon!

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