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Author Topic:   Counter-Intuitive Science
Panda
Member (Idle past 3712 days)
Posts: 2688
From: UK
Joined: 10-04-2010


Message 91 of 182 (600290)
01-13-2011 6:44 PM
Reply to: Message 66 by Dr Adequate
01-12-2011 9:50 PM


Dr A writes:
Why not?
Weight is not affected by velocity.
You can't increase your weight by moving quickly (unless you approach the speed of light maybe?).
Edited by Panda, : added question mark

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dwise1
Member
Posts: 5930
Joined: 05-02-2006
Member Rating: 5.8


Message 92 of 182 (600292)
01-13-2011 7:19 PM
Reply to: Message 90 by RAZD
01-13-2011 6:33 PM


Re: tripping stoplight sensors with a bicycle
Yes, signal loops.
In an incident similar to Dr.A's, I was driving my Ford Fiesta (this was circa 1981) and was the only car in the left-turn lane. The signal loop never picked up on the fact that I was there. After being missed a couple times, another larger car pulled up behind me and we got the green arrow.
Was my car too small to be detected? It had always worked at other left-turn lanes as well as at that one at other times. It was also in the dead of a North Dakota winter (Grand Forks, which would get the same Arctic air mass as International Falls, MN, commonly the coldest spot in the US), but I don't know whether that could have been a factor.
Back to Dr.A's problem, Wikipedia describes three types of motion detectors: passive IR, active ultrasonic, and active microwave. The last two appear to use the Doppler effect, though they could also just be timing the returns to get the range of the strongest return and comparing it with the previous return. In either case, jumping up and down should have minimal effect on radial velocity.
Edited by dwise1, : back to the problem ...

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Iblis
Member (Idle past 3895 days)
Posts: 663
Joined: 11-17-2005


Message 93 of 182 (600300)
01-13-2011 8:08 PM
Reply to: Message 88 by RAZD
01-13-2011 6:28 PM


there is a green blob ... or is it the green goblin?
That's obviously Galactus.
Not the classic original, with the giant purple costume with donkey ears; but rather the new updated one as seen in Ultimate Extinction and Rise of the Silver Surfer
Edited by Iblis, : fix title

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RAZD
Member (Idle past 1404 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 94 of 182 (600302)
01-13-2011 8:16 PM
Reply to: Message 92 by dwise1
01-13-2011 7:19 PM


Re: tripping stoplight sensors with a bicycle
Perhaps the sensor was turned down (to prevent the door opening for dogs etc), and it probably used an average over a set time period usually taken to walk along the sensor area (to eliminate people walking across the sensor area tripping the door).
That's my guess, as it would eliminate light weight and short impacts of momentum.
enjoy.

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dwise1
Member
Posts: 5930
Joined: 05-02-2006
Member Rating: 5.8


Message 95 of 182 (600308)
01-13-2011 8:52 PM
Reply to: Message 94 by RAZD
01-13-2011 8:16 PM


Re: tripping stoplight sensors with a bicycle
For my car story or Dr. A's story? I'll assume the latter.
The information we're missing is: what kind of sensor was the door using?
Everybody else seems to be assuming a pressure pad sensor, that would detect the weight of someone stepping onto it. Growing up, that's what I always saw being used at supermarkets. But to be honest, I don't recall having seen one in many years.
Instead, I always see a sensor unit mounted on the center of the lintel pointing out towards approaching foot traffic. From what I've read, that sensor could be passive IR (which I deem unlikely) or some kind of active ranging sensor (much more likely) using either ultrasonics or microwaves -- I would deem ultrasonics to be more likely. I'm not really up on capacitive field technology, but I doubt whether it would have the range to be used for this purpose. None of those technologies would be affected by the weight of the person, but rather by his cross-section and reflectivity (for active ranging) or ability to emit IR radiation.
Now, preventing false triggers is desirable, more so in alarm systems than in door openers. Sensitivity could have been turned down and he somehow slipped under that threshhold. Or else measures to eliminate false triggers could have been incorporated in the sensor and he somehow slipped through that algorithm.
But knowing what kind of sensor that door used would help in figuring out what had happened.
Edited by dwise1, : saw, not say

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Panda
Member (Idle past 3712 days)
Posts: 2688
From: UK
Joined: 10-04-2010


Message 96 of 182 (600312)
01-13-2011 9:09 PM
Reply to: Message 95 by dwise1
01-13-2011 8:52 PM


Re: tripping stoplight sensors with a bicycle
dwise1 writes:
But knowing what kind of sensor that door used would help in figuring out what had happened.
If you let your imagination 'run riot', there seems to be too many possibilities.
The sliding doors were on a tube train?
The sliding doors were opened by a security guard inside?
The sliding doors were locked because they led to a shop which closed after the previous people entered?
The sliding doors were on a lift?
etc...

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 97 of 182 (600320)
01-13-2011 11:31 PM
Reply to: Message 91 by Panda
01-13-2011 6:44 PM


Weight is not affected by velocity.
But obviously such a device doesn't measure weight as such. If you hit a set of bathroom scales with a sledgehammer, does it just register the weight of the hammer?

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Panda
Member (Idle past 3712 days)
Posts: 2688
From: UK
Joined: 10-04-2010


Message 98 of 182 (600360)
01-14-2011 6:03 AM
Reply to: Message 97 by Dr Adequate
01-13-2011 11:31 PM


Dr Adequate writes:
But obviously such a device doesn't measure weight as such. If you hit a set of bathroom scales with a sledgehammer, does it just register the weight of the hammer?
'Logic problems' often rely on incorrect interpretation of words for them to work.
e.g. "What walks on 4 legs, then 2 then 3?" is intentionally misleading as '3 legs' includes a walking stick.
I was thinking that the use of the word 'weight' was the 'trick'.
Obviously, in reality, pressure mats are triggered by any type of downward force (including weight and momentum).

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dwise1
Member
Posts: 5930
Joined: 05-02-2006
Member Rating: 5.8


Message 99 of 182 (600395)
01-14-2011 10:51 AM
Reply to: Message 96 by Panda
01-13-2011 9:09 PM


Re: tripping stoplight sensors with a bicycle
So then he wasn't describing an actual event, but rather presenting a contrived puzzle? OK, lost my interest.
But seriously, where are all you guys still seeing pressure pad door sensors being used?
Edited by dwise1, : but seriously, ...

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 100 of 182 (600399)
01-14-2011 11:09 AM
Reply to: Message 66 by Dr Adequate
01-12-2011 9:50 PM


I had better tell everyone so that the course of the thread can be resumed.
I distinctly said that my leap was vertical; and I said that hoped the force with which I came down would be sufficient to open the door.
But if the force with which I came down was sufficient to open the door, then so would be the equal force with which I went up, and the door would (as in fact it did) open when I left the ground, not when I struck it.
This all goes to show, I think, that the simplest aspects of physics can elude the intuition of the cleverest people.

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Panda
Member (Idle past 3712 days)
Posts: 2688
From: UK
Joined: 10-04-2010


Message 101 of 182 (600410)
01-14-2011 11:49 AM
Reply to: Message 100 by Dr Adequate
01-14-2011 11:09 AM


Dr A writes:
I distinctly said that my leap was vertical;
You distinctly said the sensors detected your weight too.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 102 of 182 (600421)
01-14-2011 12:20 PM
Reply to: Message 101 by Panda
01-14-2011 11:49 AM


Judging that the sensors weren't sensitive enough to detect my modest weight, I decided to leap vertically in the air in the hope that when I came down the extra force of my impact would open the doors.

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Iblis
Member (Idle past 3895 days)
Posts: 663
Joined: 11-17-2005


Message 103 of 182 (600438)
01-14-2011 1:16 PM
Reply to: Message 100 by Dr Adequate
01-14-2011 11:09 AM


Christ, is that all? Remind me never to wait for your other shoe to drop ever again.
slevesque got the key point already at Message 79, though no one realized you would be so obtuse as to be sticking on the point of when the door opened rather than meaning that it didn't.
This one tells us more about morbid psychology than it does about physics.

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onifre
Member (Idle past 2950 days)
Posts: 4854
From: Dark Side of the Moon
Joined: 02-20-2008


Message 104 of 182 (600442)
01-14-2011 1:31 PM
Reply to: Message 10 by Larni
01-10-2011 10:42 AM


That heavy things fall to earth the same speed as lighter things.
I don't think that's right, Larni. Falling to earth is affected by air resistance, so it would slow the lighter object down. I think you mean in a vacuum they "fall" at the same rate?
- Oni

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DrJones*
Member
Posts: 2284
From: Edmonton, Alberta, Canada
Joined: 08-19-2004
Member Rating: 6.8


Message 105 of 182 (600453)
01-14-2011 2:28 PM
Reply to: Message 104 by onifre
01-14-2011 1:31 PM


Falling to earth is affected by air resistance, so it would slow the lighter object down.
Air resistance is dependant on the shape of the object not the mass. So a properly configured heavy object could fall slower than a lighter one.
But you're right about it needing to be a vaccum (or two identically shaped objects) to show that acceleration due to gravity is not related to the masses of the objects.

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