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Old Jan 2nd 2018, 12:47 PM   #1
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Does this show atmospheric pressure or air resistance?

Please help a discussion we're having here. The demo in question is attached.

I had to do this demo as the very first introduction to pressure for K8 students. It's supposed to show that there is atmospheric pressure. I don't see it being much to do with pressure as much as air resistance. Surely air pressure is taken as static and therefore as soon as the paper/ruler/paddle starts to move, it's all about air resistance?

Please let me know what you think
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Does this show atmospheric pressure or air resistance?-pressure-demo-1.png  
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Old Jan 2nd 2018, 12:56 PM   #2
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The point of this experiment is that the board, with the paper over it hardly moves as the board breaks. Since it hardly moves, it cannot well be "air resistance" in the usual sense- resistance to motion through the air. The point is that the board on the bench is held down by a force equal to the air pressure multiplied by the area of the newspaper.
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Old Jan 2nd 2018, 01:50 PM   #3
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The board tries to lift the paper faster than the air can get in under the paper to fill the gap.
This creates lower pressure under the paper than on the top.
This pressure difference holds the board in place.
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Old Jan 2nd 2018, 02:22 PM   #4
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Thanks, HallsofIvy, but you can't tell me it's not about air resistance as you contradict yourself. If it moves, it's air resistance, any pressure differential is as a result of this. That's the same logic as saying that a long, long lever isn't levering because the object hardly moved .... the longer the lever, the less the movement, therefore, if the paper/card hardly moves, it's showing how effective air resistance is?
Thanks, Woody, I like your idea drawing on Bernoulli's ideas but that's not the point.
Is this an effective way of introducing students to the concept of what atmospheric pressure is or is it merely air resistance misinterpreted?
How can this be effective if the very physics of what it's supposed to show are misinterpreted?
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Old Jan 2nd 2018, 02:43 PM   #5
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What do you think the mechanism of air resistance is?

In this case it is due to air pressure, just the same as in a boat sail.

If you don't like this experiment, try blowing a door shut with a wind artificial or natural.
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Old Jan 3rd 2018, 11:11 AM   #6
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This experiment would work with a pivot in space ... i.e. with nothing for the paper/ruler combination to rest on and at any angle .... and the reason is that it's due to air resistance, not atmospheric pressure ... think about it .... if you invert the whole thing and had to slap the ruler upwards, or turned it vertically .... the experiment would still work due to air resistance, not air pressure. If the experiment works at every angle and orientation, it's independent of atmospheric pressure, surely?
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Old Jan 3rd 2018, 12:51 PM   #7
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Did you say you were teaching Physics?

What would be your response to a student who failed to answer you when you asked a question specifically designed to help him?

I repeat

What do you think the mechanism of air resistance is?
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Old Jan 3rd 2018, 01:39 PM   #8
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You repeat?

I think I'll ignore you from now on as you're obviously a petulant egoist ...

I would suggest that you need more patience and a less hubristic attitude if you're going to get a polite response from me.

My response to a student would be how can I help you understand this more. I assume yours would be to repeat yourself, only louder?
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Old Jan 3rd 2018, 01:44 PM   #9
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The mechanism of air resistance would involve instantly changing the surface pressure one side of the paper while reducing it on the other, which if on a desk, would create a pressure differential and a suction effect. However, this experiment would work at any orientation so the suction idea relies on the paper/ruler combination being over a solid surface. If you're changing the pressure at either surface you cannot have atmospheric pressure around these surfaces, you're adding or subtracting due to air resistance. None of which provides a direct link between the result of the experiment and the reason stated, nor does it explain atmospheric pressure accurately. I guess in this post-truth era, we can even excuse an experiment that doesn't do what it's stated to prove.
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Old Jan 3rd 2018, 01:48 PM   #10
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Attacking the messenger will not alter the fact that you do not understand the mechanism of air resistance (or air pressure for that matter).

Last edited by studiot; Jan 3rd 2018 at 01:50 PM.
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