Author Topic: Episode #253  (Read 13963 times)

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Offline Citizen Skeptic

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Re: Episode #253
« Reply #120 on: June 04, 2010, 06:00:57 PM »
If you strike the water, the force it requires to force your foot into it is the same force pushing back on your body. That's the relevant law of physics that makes it possible, not impossible.
This is what always confuses me. So if I were to drop a brick into the water, there would be an instant where the brick came to rest on the surface of the water before sinking?
Why do you think that assumption follows from the quote? The fact that the brick suddenly starts to feel a new force opposing its motion as it hits the water does not mean it stops moving at that point. The force would simply cause the brick to start to experience an additional deceleration.
I wasn't sure it did. I'm just trying to get my head around this. All I'm getting is a headache! :)

OK. Ignoring air resistance, as the brick falls through the air the only force on it will be its own weight, so it will accelerate at g (= 9.81 metres (about 32ft) per sec2). When it hits the water the net force on it will be its weight downwards, less the upthrust from the water due to buoyancy (Archimedes' Principle), less the viscous resistance of the water (which depends on speed, shape, etc). Whatever the details, the net force will now be less than its weight, so it will accelerate at less than g. In fact, if the brick is fully buoyant (e.g. made of wood), or if the viscous resistance is high enough, it will actually start to decelerate at this point.

Either way, its velocity will not change abruptly at the instant it hits the water (and it certainly won't stop altogether), but the rate of change of the velocity will change as a result of the altered net force.

OK, now I get it. Thank you kind sir! :)
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Offline elert

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Re: Episode #253
« Reply #121 on: June 06, 2010, 02:32:18 PM »
From the 2008-2009 report of the President's Cancer Panel released 6 May 2010.

Quote
At this time, there is no evidence to support a link between cell phone use and cancer.
<http://deainfo.nci.nih.gov/advisory/pcp/pcp08-09rpt/PCP_Report_08-09_508.pdf>

The report contradicts itself later on when it describes ways to minimize exposure to cell phone radiation. It also discusses the potential risk of exposure to em radiation from power lines.

Read my assessment of the physics here.
« Last Edit: November 14, 2010, 03:40:32 PM by elert »

Offline Citizen Skeptic

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Re: Episode #253
« Reply #122 on: June 06, 2010, 03:20:50 PM »
@elert - great write-up. and welcome!  :)
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Offline Swagomatic

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Re: Episode #253
« Reply #123 on: June 09, 2010, 05:52:12 PM »
@elert - great write-up. and welcome!  :)

Seconded!
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Offline Paperclip.

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Re: Episode #253
« Reply #124 on: June 18, 2010, 12:04:53 AM »
Re: Halley's Comet
I was flicking through a book I got the other day on astronomy and came to the section on comets.  Quote, "In April 1910, Comet Halley appeared in Earth's skies and was widely observed.  The following appearance, in 1986, disappointed a lot of people, who had been led to believe it would be as good as the 1910 show.  The problem was that many of the reports of Comet Halley from 1910 had become confused with those of another 1910 comet, the Great Day-light Comet, which had put on a good show in January of that year.  It was so bright that it could be seen in daylight, hence the name, and far outperformed poor old Halley.  This confusion, combined with unfavorable angles and distance in 1986 - as well as more light pollution in cities, which makes it harder to see comets - was what led to a general disappointment in the community."

I'd never heard of that before now, so...
"...apparently the rest of you do not, or you are not yet sufficiently well educated to realize that authority, the authority of a reputation or the authority of a printed page, means very little. All of you should hope to someday develop as much insight and persistence as Mr. Alford."

Hardy Cross

Offline Trinoc

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Re: Episode #253
« Reply #125 on: June 18, 2010, 05:09:16 AM »
While on the subject of comets, who is this girl Hayley who had a comet named after her? Was she Halley's sister? Perhaps one of her descendants works on the Large Hay Dron Collider.
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Offline Hampster

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Re: Episode #253
« Reply #126 on: June 21, 2010, 02:17:28 PM »
Awesome @elert  I brought up a couple of other points to add to the whole debate here:  http://skepticaltreehugger.org/2010/06/cell-phony-reasoning

--Dave
WARNING- Opinions subject to change as new evidence emerges.