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Old Jan 3rd 2018, 03:01 PM   #11
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I wasn't attacking the messenger, just your crappy attitude

As for you stating that I don't understand the mechanism of air resistance, I haven't got round to saying what my understanding of it is, merely suggesting what may happening in this experiment that means it does not explain atmospheric pressure. I might suggest you change your attitude of being patronizing until being called out on it and then making false statements regarding what I've previously said to distract from that. As a Brit teaching in the USA, your response fits in perfectly with the US view of the English.

And, please, enlighten me with what your ideas might be. As a former fighter pilot and a curious (hence the alternate way of viewing this experiment) scientist (someone with an open mind), I'm really looking forward to hearing what you have to say.
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Old Jan 3rd 2018, 03:22 PM   #12
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Studiot
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.
I see nothing controversial or personal about any of the three sentences in my first post, you were so rude about.

In fact the first sentence was a question that did exactly what you said I should do.
Seek to find out how I could help you.



HallsofIvy has already supplied the exact and correct explanation of the course of the experiment.

I will expand a little.

Prior to the impact the spread out surface of the newspaper is subject to standard air pressure.
Nothing is moving so the downward force is calculated as HOI stated.

On impact the ruler attempts to lift the paper against this force.

It fails to lift the paper and so bends rapidly down against the table edge.

The ruler is therfore subject to an impulsive bending force which is sufficient to break it as it exceeds its bending strength.

The bending strength is much much lower than either the tensile or compressive strength.

For your information.

Air resistance can be the result of air pressure as applied in this way or it can be as a result of air friction.

Since the friction part increases with velocity it plays no significant part here.

Further I assume you know that there is no such physical phenomenon as suction, anywhere in the known universe.

Edit

In the Youtube video of the experiment I watched, the demonstrator took great pains to stress that the blow must be rapid and clean.
He even demonstrated what happens when the blow is softly delivered - The ruler lifts the paper and does not break, although it must be subject to the same resisting forces.
However the demonstrator did not mention the impulsive forces by name or that they can be many times the applied force, depending upon the impulse integral.
In fact the explanation offered was brief in all departments.

Last edited by studiot; Jan 3rd 2018 at 03:47 PM.
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Old Jan 3rd 2018, 07:23 PM   #13
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This is fascinating! In case you think I was being rude, not at all ... just not prepared to be communicated with like you initially did.

It's fascinating, everyone giving the same old answers because ... what? Think about it, you're telling me that you think the paper is held down by atmospheric pressure? Even at rest, this isn't the case. As soon as it becomes a dynamic system, especially without the solid, suction forming bench, it's air resistance that's going to break the lever.

And what's really fascinating is ... that I'm asking this on behalf of a science polymath student whizzkid who asked me about after the demo. So I said I honestly wasn't sure what other effects could be involved that explained atmospheric pressure in the direct way everyone seems to blindly accept.

Your responses seem to ignore what I've already mentioned that, if you take away the solid surface beneath the paper/board lever, it seems to be even less connected with atmospheric pressure.

You say that HallsofIvy gave a perfectly good explanation. Not really because both the whizzkid and I both looked at your responses and said almost the same thing ...

"They think that it's atmospheric pressure holding down the paper? Haven't they heard of gravity?"
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Old Jan 3rd 2018, 10:26 PM   #14
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BTW, FYI, your suggestion of blowing a door shut is an example of wind, caused by pressure differentials. Atmospheric pressure is not this (I won't patronize you with the various definitions), which is what this experiment claims to show. At most, this experiment can claim to show the relationship between area and force.
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Old Jan 3rd 2018, 11:47 PM   #15
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I agree with all the 3 answers offered ....

The paper is resting on the ruler , held down by gravity. Atmospheric pressure pushes all around the paper up and down .

When the ruler is struck the paper will start to move up slightly , wanting to suck air in around the edges to equalize pressure , but this is prevented by the paper edges being held down to the table by the partial vacuum created. So it is atmospheric pressure which causes this effect ..

You can prove which theory is correct by experiment ...you will have to create a table which is strong , but has many holes drilled in it .... or better a strong metal grating .



Make sure the grating is not resting on a flat surface , preventing air movement.
This will allow air to come in from below , preventing the partial vacuum being created , If your idea that it is air resistance is true there should be no difference to the solid table.
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Old Jan 4th 2018, 04:52 AM   #16
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First of all let me thank ITYSON for introducing this experiment for discussion.
I , for one, had not come across this particular one before.

I am trying to tell you that air resistance and air pressure are aspects of the same thing.

Air resistance is a Force in the true meaning of the word.

Air pressure is a distributed force, which can (and does in this case) result in a force on the ruler, additional to the gravitational body force that always acts.

Of course, air pressure is cause by gravity,

But then gravity is caused by the Earth's mass

And the Earth's mass is caused by its chemical composition

And the individual masses of the chemical is caused by....
Just how far do you want to go?


As well as myself, many here seem to find the experiment interesting and there is much to discuss about it.

Oz93666 has offered an excellent variation.

Another variation is to proceed as follows.

Set the ruler up under the newspaper.
Instead of hitting it with your hand use a heavy piece or wood or a hammer, but first
Hang the striker gently from the end of the ruler under its own weight.


What happens?
Does the ruler break?


Anchor the ruler firmly for example under some bricks.
Hang a weight pan on the cantilevered end.
Add weights until the ruler breaks and note the breaking load.


Why is the breaking load much greater than the weight of the striker?
Why does the ruler always break at or close to the overhang point, not at the load point?
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Old Jan 4th 2018, 01:48 PM   #17
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To teach K8...

Sir,

Thought-provoking scenario to explain to K8s. Surely the "demo" will not work for
all board/newspaper/impact-orientation possibilities. When the board does break..
How to CONVINCE k8 that's atmosphere?

Also gases have the property "pressure" but cannot be measured with regard to "resistance."

For class about atmospheric pressure, perhaps make a simple "air pump." Here is an idea.

Otto Von Guernike | THERMO Spoken Here!

Wilma's Surge | THERMO Spoken Here!

Good Luck, JP
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Old Jan 6th 2018, 05:16 AM   #18
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I think there are actually a couple of effects both happening here.
As the ruler starts to lift the centre of the sheet of paper, it will generate a low pressure region under the paper, and the pressure difference will act to hold the paper down.
Also, as the paper starts to lift it will be pushing the air above out of the way,
the viscosity of the air will resist this, and so generate another force also acting to hold the paper down.

Both these effects will be lost if the ruler is moved slowly.
The air will have time to move to equalise the pressure difference below the paper
and also to "get out of the way" above the rising paper.

If you want to know which effect is dominant, get your calculator out.
I would guess that the pressure difference dominates, but I'm far too lazy to actually sit down and work it out.
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Old Jan 7th 2018, 01:25 AM   #19
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Originally Posted by ITYSON View Post
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
Keep in mind that there are two different kinds of pressure. There's static/fluid pressure, like that which acts on a diving bell deep in the ocean and there's ram pressure, the force per unit area caused by moving air on a body.
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