I ate out yesterday under a fan and noticed that the wax paper and foil that we got with our food kept blowing away if we didn’t put something on it. Regarding this, I had two questions pop into my mind.
1) Why does the paper/foil blow away sideways even though the fan is directly above us?
2) Why is it that if we crumple the paper/foil it doesn’t move anymore? The weight obviously doesn’t change, so why does it suddenly gain the power to resist the air blown by the fan?
Thanks!
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1, larger surface area and low weight means it gets caught in the wind and blows away easier. 2, A crumpled ball will still roll away but it can’t fly cause it doesn’t have enough surface area to actually get caught in the wind. It’s the same weight yes. But more densely packed into a sphere than a flat piece of paper
If the fan is blowing down, once the air hits the table it has to go sideways. So its going to flow across the paper causing aerodynamic lift. Crumpled up it has much less surface area and surfaces at many different angles to the air stream so less concerted force in a particular direction
Paper sheets have a lot of area relative to their mass. Any kind of draft or turbulence will tend to pick them up. Since their mass is evenly distributed they tend to tumble around instead of just flying edge-first into the ground (unlike e.g. a paper airplane which should always be nose heavy i.e. have its center of gravity in front of its center of lift.).
The copy paper I have here only weighs 80g per square meter.
If you fold it in half twice it will have a fourth of the surface area, so its “blow-ability” will be quartered. Crumple it into a ball and it will probably have far less surface area than that. An A4 sheet of paper has 623.7m² of surface area (one side). How big is when you turn it into a ball? Maybe 10cm in diameter? That’s only 31cm² of cross sectional area (a 20-fold reduction!) while the mass stays the same.
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