Sticky card! Application of Bernoulli / Pressure incorrect?

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Homework Statement:

punch a hole with a nail through a plastic card, place a spool at the end of it and suck through it. What do you observe? Why does that happen?

Relevant Equations:

Bernoulli's equation.
After doing what the instruction did, I noticed that the card stuck to the spool, and generated the hypothesis that it was the same effect as when putting two papers together and blowing through them, the lower pressure of the faster speed of the wind makes the higher pressure outside move the two papers together, but after trying the same experiment without punching a hole I got the same result, and my teacher insisted that the "Why" part of this problem used Bernoulli, so I have two questions I need help on:

How does Bernoulli's equation apply in this problem?
How does the hole affect the problem?
 

Answers and Replies

  • #2
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Welcome, Fuux!

How did you blow without the orifice?
Could you post a diagram for both situations?

What the Bernoulli's principle states?
 
  • #3
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Thank you!

I think I was able to just inhale the air inside the spool and I think that was enough for the card to stick, but I guess I could have inhaled some air from some space between the spool and the card.
As for a diagram I was given the photo attached.

Bernoulli's principle states that pressure in a fluid goes down when its speed increases. I could imagine that His Molecular Kinetic Theory could perhaps indicate that lower amount of air particles on the spool when I inhale means less pressure exerted from these moving particles against the card, but I believe that's more of a chemistry answer and I cant see the relation with the formula my teacher talked about.
 

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  • #4
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Sorry, I have never seen this experiment done by sucking from the spool, but by blowing trough it and against the card.
That blowing creates a radial air flow between the spool and the card, depressing the static pressure on that interphase.

I can only guess that you could create the same effect by strongly sucking from the spool (leaving some gap between spool and card), except when the central hole is so big that more air goes through it than radially.

bernoulli-spool-diagram-2-300x283.png
 
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this was very useful anyway, Thank you!
 

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