Motion of a Door as I Walk by it Quickly (no contact)

[Richie Smash]

Hey this is just a random question, but recently I've started studying physics, only last year.

Before this I never used to question really a lot of things i took for granted, but now I see the world with a different light although I'm still quite the novice.

Something that used to make me think, when I walk past my bathroom door really fast, it will swing on it's on towards me. I make no contact with the door, as I tested it a few times.

My first assumption was it had something to do with friction of my body and air particles causing them to go into motion and collide with the door at a Force (N). But if this was so, wouldn't the door swing away from me?

Does anyone have any ideas on this? I think it's quite interesting.

 

[anorlunda]

https://en.m.wikipedia.org/wiki/Bernoulli's_principle

Congratulations on your curiosity. I may be 100% wrong on this because I can't visualize exactly what you are describing, but it could be the Bernouli Principle. Read up on that.

 

[Richie Smash]

https://en.m.wikipedia.org/wiki/Bernoulli's_principle

Congratulations on your curiosity. I may be 100% wrong on this because I can't visualize exactly what you are describing, but it could be the Bernouli Principle. Read up on that.

Wow, I'm probably going to have to spend days reading this, but I won't give up, I'm a page in and suddenly it got complex and my brain is struggling to keep up :D

 

[sophiecentaur]

https://en.m.wikipedia.org/wiki/Bernoulli's_principle

Congratulations on your curiosity. I may be 100% wrong on this because I can't visualize exactly what you are describing, but it could be the Bernouli Principle. Read up on that.

.. . . or it could be that the doors are closed in the part of the corridor you are walking towards and you are acting like a piston in a closed cylinder, increasing the pressure in the closed end of the cylinder as you move - pushing the bathroom door. The hinges on the bathroom door must be particularly free. I suggest that the ceiling could be fairly low so that your body take-up a significant proportion of the cross sectional area of the corridor (minus the area of that door.

 

[russ_watters]

It's complicated and at least Bernoulli adjacent, but the way I would describe it is that due to your poor aerodynamics, you pull a substantial wake behind you (and push air in front of you), causing an area of low pressure that pulls the door toward you after you pass.

The piston effect can amplify it the pressure change, but cars do the same thing in free air. That's why debris next to a car (like leaves) ends up on the road behind it.

 

[sophiecentaur]

It's complicated and at least Bernoulli adjacent, but the way I would describe it is that due to your poor aerodynamics, you pull a substantial wake behind you (and push air in front of you), causing an area of low pressure that pulls the door toward you after you pass.

The piston effect can amplify it the pressure change, but cars do the same thing in free air. That's why debris next to a car (like leaves) ends up on the road behind it.

Thing is, he's not going faster than about 1m/s. If he could experiment with opening and closing other doors to see if the 'piston effect' is feasible. . . . .?

 

[anorlunda]

I think the key to finding the answer could be timing. @Richie Smash , can you do it again and tell us where your body is when the door starts to move? Are you approaching the door? at the door? or already past the door?

 

[anorlunda]

I just had another idea. @Richie Smash , maybe we can make a scientist out of you.

You could do the following experiment at home. Take some sheets of ordinary paper. Hang each sheet from a thread and tape it to the ceiling of the hall such that the paper is parallel to the walls. Then set your phone to record video, and walk through the hall. The video will show the papers moving as you pass. You will be able to see which direction they move and when. You might be able to use hanging threads with no paper, if the threads are visible on the video.

What science will you learn from that? Most important is the scientific method; first gather reliable evidence, then think about what the evidence means. That is so much better than a verbal description of the door moving that is so hard to explain accurately. The methods of collecting evidence are more valuable than the answer to the riddle, why do they move?

 

[Richie Smash]

Hi guys, thanks for all your replies.
To answer your question anorlunda, it moves when I'm past the door.
What I'll do, is measure the length and width of the hall, which has 3 doors, and I'll draw a diagram of this hall, with the location of each door.

And that experiment with the hanging thread and paper sounds pretty good, I'll do that, just need some time so I will get back to you guys.

But the diagram I can do this evening, is that it's currently 10:47am where I live so please hold tight.

 

[russ_watters]

If he could experiment with opening and closing other doors to see if the 'piston effect' is feasible. . . . .?

That wouldn't be analagous; doors seal, so the piston effect works well. A person in a hallway makes a bad piston.

Better tests would be;
1. Try to hug the wall on the other side of the hallway and see if it still happens.
2. Try it with doors not in a hallway.

 

[sophiecentaur]

A person in a hallway makes a bad piston.

But also a person walking slowly may not be generating much a bernoulli effect. Changing the situation with the other doors would make a big difference to how effective a piston the moving person is. (Closed cylinder / open ended cylinder pressure changes)
Would you expect the same effect in a large room with 'infinite cross sectional area? I don't think I would. An experiment would help to resolve this.

 

[anorlunda]

And that experiment with the hanging thread and paper sounds pretty good, I'll do that,

I like this.The advisers are all saying "experiment" and Richie is saying, "I'll experiment." We're in agreement.

Take your time Richie. No hurry.

 

[FactChecker]

I agree with all, that more experiments would be nice, but the results may be mixed and confusing. @russ_watters 's hypothesis that it is "drafting" as in a car behind a large truck seems most likely to me. The piston theory requires sealed air path down the hall and around corners, which seems less likely. The drafting theory only requires local effects. The force of the "drafting" turbulence behind a moving object is often underestimated. I see trucks and trailers all the time with streamlined front ends but squared-off rear ends. If you've ever ridden a motorcycle close behind a truck, you know that the force is significant.

 

[sophiecentaur]

The piston theory requires sealed air path down the hall

That's what I was assuming (all other doors closed). I could imagine the piston effect working in a small, low hallway, already half obstructed by an open door, almost to the ceiling. A human body would take up quite a significant proportion of the area. But we'd need to know the dimensions and the speeds involved.

 

[Richie Smash]

Hi Guys, Here is a very poor diagram of the hall way each door and the door in question D4. The measurements are approximations for convenience sake.

Observations:

With each door closed besides D4, walked past at a brisk pace at about 1m/s the door swung inwards each time for each attempt.

The door swung inward as I was aligned with it, almost past it but not quite, about when my body was halfway past the doors width, and the same thing happened when I tried to hug the wall as much as possible, this is the wall inside the door's frame, (not shown in diagram.)

I repeated this about 3 times for each scenario, and I got the same result whether D1, D2 and D3 were closed or open.

The exact same thing happened with each door OPEN, as next to D3, is a railing and a large open space going downstairs, I suspect this has something to do with it.

The only walls are marked W1 and W2

Now about the paper hanging experiment, I can probably do that tomorrow, so I will keep posting my observations.

NOTE: THE height of the hallway is about 3 METERS

ALSO: I tried the same experiment with D3, which leads to my room. My room is shaped like a cuboid very uniform and a bigger volume than the hall or the bathroom itself, and to my surprise it did not happen.

I then tried the experiment with a door leading to another bathroom, from a room with a much bigger volume to the other bathroom, which is more rectangular shaped and has a smaller volume, and the door swung inwards again, I don't know if this is some valuable insight but I thought I'd share.

 

[Richie Smash]

HERE IS AN UPDATED DIAGRAM

 

[Altair Tans]

You walk past and drag some of the air with you out of the room.....hence pressure on the inside of the room is more than that near the door.....air particles on the inside side of the door outnumber the ones on the outside side which causes a net force out of the room which pushes the door out of the room causing it to close

 

[lekh2003]

I think this seems to be a simple case of Bernoulli's principle. This happens to me all the time, the velocity of the air on one side generates a pressure differential and the door moves.

If I'm right, this is one of the ways aeroplanes generate lift. The air on the top flows faster than on the bottom and the aeroplane begins to rise.

 

[sophiecentaur]

This thread reminds me of the "How does an aeroplane wing work?" discussions. The Bigendians say it's Bernoulli and the Littleendians say it's Newton 3. Neither is wrong and neither is exclusively right. In this case, the air flowing past the moving body is driven by a pressure difference (Newton 1) and we are only arguing about what is causing that difference, a dynamic effect of the passage through the air or a quasi static effect of displacement. (piston) We could build (at least design) two systems which could work due exclusively to one or the other or a combination of the two. Here, the numbers are important and the diagram that's been supplied is half way there. How high is the corridor, how wide is the door and how fast is 'walking'?

 

[FactChecker]

It's neither a pressure build up from a "piston" action, nor Bernoulli. The stairs shown in the updated diagram of post #16 rules out pressure build up (which I thought was unrealistic except in a sealed, air-tight corridor anyway). Bernoulli is also the wrong way to look at this problem. The turbulence and air flow behind the person walking past is the cause. Trying to analyse this problem in more detail is virtually hopeless. Even relatively simple situations require wind tunnel experiments to verify analysis.

 

[Richie Smash]

This thread reminds me of the "How does an aeroplane wing work?" discussions. The Bigendians say it's Bernoulli and the Littleendians say it's Newton 3. Neither is wrong and neither is exclusively right. In this case, the air flowing past the moving body is driven by a pressure difference (Newton 1) and we are only arguing about what is causing that difference, a dynamic effect of the passage through the air or a quasi static effect of displacement. (piston) We could build (at least design) two systems which could work due exclusively to one or the other or a combination of the two. Here, the numbers are important and the diagram that's been supplied is half way there. How high is the corridor, how wide is the door and how fast is 'walking'?

Well the corridor is approximately 3m high, the width of the door is 0.75m and my speed my best estimate is about 0.85 m/s.

Also I didn't wish for this to be an argument i really have been curious about this for a long time, my original hypothesis being Newton's 3rd law as the door would push back with an equal force to the force of the air being pushed against it. I definitely have not reached AP physics, but in my head I'm picturing some sort of air swirling behind the door, kinda like when a car speeds and the leaves fly, you can see a sort of swirling pattern of the air, that would be my best guess, but unfortunately I cannot get more technical and for that I do apologize :(.

I just thought I would share to gain some insight because I enjoy learning.

 

[sophiecentaur]

Also I didn't wish for this to be an argument

Lol. Never post on PF if you don't want to trigger an argument, my friend.

Newton's 3rd law as the door would push back with an equal force to the force of the air being pushed against it.

That is certainly not an answer as N3 says nothing of the sort. You need to read about Newtons Laws and how they actually affect things.

I'm picturing some sort of air swirling behind the door,

That's much more like a good reason.

 

[FactChecker]

Also I didn't wish for this to be an argument

This discussion really says that your question was a good one.

 

[Altair Tans]

Well the corridor is approximately 3m high, the width of the door is 0.75m and my speed my best estimate is about 0.85 m/s.

Also I didn't wish for this to be an argument i really have been curious about this for a long time, my original hypothesis being Newton's 3rd law as the door would push back with an equal force to the force of the air being pushed against it. I definitely have not reached AP physics, but in my head I'm picturing some sort of air swirling behind the door, kinda like when a car speeds and the leaves fly, you can see a sort of swirling pattern of the air, that would be my best guess, but unfortunately I cannot get more technical and for that I do apologize :(.

I just thought I would share to gain some insight because I enjoy learning.

Knowing exact motions of the air particles would be a godlike achievement...
.

So instead we talk in the language of pressures...

More density of particles = More pressureBehind the door (in the room) there is high density while in front of door (outside the room) while walking you took away some particles causing low densityIn normal conditions on both sides of door no of particles colliding per unit time is same hence it goes neither way but in this case collision from behind is more than from front hence i guess you are able to imagine what's happening nowThe air particles are playing something like a tug of war where instead of "pulling" on a "rope" particles are "pushing" onto a "door"

So what your role here is that while walking past you took away some of the players from your side and hence decreased the strength causing the other side to winI would suggest at this stage you imagine every phenomenon around you by connecting it to something in everyday life.....build up a story.....have fun with physics...make scenerios your friends maths your toys and your mind your playground...talk play and have fun!

 

[Richie Smash]

Knowing exact motions of the air particles would be a godlike achievement...
.

So instead we talk in the language of pressures...

More density of particles = More pressureBehind the door (in the room) there is high density while in front of door (outside the room) while walking you took away some particles causing low densityIn normal conditions on both sides of door no of particles colliding per unit time is same hence it goes neither way but in this case collision from behind is more than from front hence i guess you are able to imagine what's happening nowThe air particles are playing something like a tug of war where instead of "pulling" on a "rope" particles are "pushing" onto a "door"

So what your role here is that while walking past you took away some of the players from your side and hence decreased the strength causing the other side to winI would suggest at this stage you imagine every phenomenon around you by connecting it to something in everyday life.....build up a story.....have fun with physics...make scenerios your friends maths your toys and your mind your playground...talk play and have fun!

Your explanation really gave me an idea of what is going on now, I do feel satisfied as that has been happening for months.
I also see why I was wrong with N3, because the air particles Force on the door, would be equal to the door's reaction force.
Acceleration of the door would require an unbalanced force.
I see what helps me is to picture things visually, rather than just read everything.

 

[FactChecker]

As you walk rapidly forward, the air must flow into the empty spot behind you because "Nature hates a vacuum." When the air flows in behind you, it pushes less against the door. That allows the air on the other side of the door to push it toward you.

There can be a lot more going on, including turbulence, but they do not change the basic facts.

 

[anorlunda]

Ah come on guys. Let's let Richie do his experiment and look at his data before we drown him in conclusions.

 

[lightarrow]

Something that used to make me think, when I walk past my bathroom door really fast, it will swing on it's on towards me. I make no contact with the door, as I tested it a few times.

My first assumption was it had something to do with friction of my body and air particles causing them to go into motion and collide with the door at a Force (N). But if this was so, wouldn't the door swing away from me?

Does anyone have any ideas on this? I think it's quite interesting.

Bernoulli. Maybe you don't remember or you're too young, but in the past, cars and motorcycles had carburators: gasoline is suck up from a lower container by fast moving air on the carburator's upper pipe.
Here substitute "gasoline" with "door".

--
lightarrow

 

[jerromyjon]

As you walk rapidly forward, the air must flow into the empty spot behind you because "Nature hates a vacuum."

I'm surprised no one has mentioned Venturi...

 

[FactChecker]

I'm surprised no one has mentioned Venturi...

The suggestion of Bernoulli is related to venturi. That may be a perfectly good way of looking at this problem, but it's not simple.

 

[Richie Smash]

I just had another idea. @Richie Smash , maybe we can make a scientist out of you.

You could do the following experiment at home. Take some sheets of ordinary paper. Hang each sheet from a thread and tape it to the ceiling of the hall such that the paper is parallel to the walls. Then set your phone to record video, and walk through the hall. The video will show the papers moving as you pass. You will be able to see which direction they move and when. You might be able to use hanging threads with no paper, if the threads are visible on the video.

What science will you learn from that? Most important is the scientific method; first gather reliable evidence, then think about what the evidence means. That is so much better than a verbal description of the door moving that is so hard to explain accurately. The methods of collecting evidence are more valuable than the answer to the riddle, why do they move?

Ok, so the best I could do was, hung a piece of paper, from tape, I couldn't use a phone to record it, but i looked up while walking (while being afraid I would tumble down) and after many unsuccessful attempts, I noticed the paper swinging from East to West as I walked under it.

I apologize for not being able to carry that out in full but I did attempt.

What I learned... hmmm well I would have to say that, by me displacing air particles at a speed of about 1m/s, I was creating areas of low pressure behind me, causing a vacuum of some sort which the area of higher pressure above and behind me, rushed to fill perhaps.

This is my hypothesis, sorry if its not accurate but I think it seems reasonable.

 

[anorlunda]

Ok, so the best I could do was, hung a piece of paper, from tape, I couldn't use a phone to record it, but i looked up while walking (while being afraid I would tumble down) and after many unsuccessful attempts, I noticed the paper swinging from East to West as I walked under it.

I apologize for not being able to carry that out in full but I did attempt.

What I learned... hmmm well I would have to say that, by me displacing air particles at a speed of about 1m/s, I was creating areas of low pressure behind me, causing a vacuum of some sort which the area of higher pressure above and behind me, rushed to fill perhaps.

This is my hypothesis, sorry if its not accurate but I think it seems reasonable.

Congratulations Richie. No need to apologize. It takes years of training and lots of helpers to do professional experiments.

I think you're right about the low pressure behind you. If you re-read this thread, you will see that several of the others said that, although using different words.
There might be higher pressure in front of you, but it might be slightly harder to close that door than to open it.

But the lesson that you already learned is that science is based on evidence. We have ideas (theories) and then run experiments and see if the results are consistent with the idea. Sometimes we get experimental evidence first and then have to come up with ideas to explain it. Either way, evidence is the key ingredient in real science.

I wish you well in your future studies.