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

1. Jan 2, 2018

### 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 alot 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.

2. Jan 2, 2018

3. Jan 2, 2018

### Richie Smash

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

4. Jan 3, 2018

### sophiecentaur

.. . . 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.

5. Jan 3, 2018

### Staff: Mentor

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.

Last edited: Jan 3, 2018
6. Jan 3, 2018

### sophiecentaur

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. . . . .?

7. Jan 3, 2018

### Staff: Mentor

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?

8. Jan 3, 2018

### Staff: Mentor

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?

9. Jan 3, 2018

### 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.

10. Jan 3, 2018

### Staff: Mentor

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.

11. Jan 3, 2018

### sophiecentaur

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.

12. Jan 3, 2018

### Staff: Mentor

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.

13. Jan 3, 2018

### 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.

14. Jan 3, 2018

### sophiecentaur

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.

15. Jan 3, 2018

### 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.

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16. Jan 3, 2018

### Richie Smash

HERE IS AN UPDATED DIAGRAM

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17. Jan 3, 2018

### 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

18. Jan 4, 2018

### 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.

19. Jan 4, 2018

### 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'?

20. Jan 4, 2018

### 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.