Greatest physical misconceptions

[epoisses]

What about a thread on the greatest fundamental physical misconceptions. They can be historical or present-day. I'll kick off with a real life example.

At home we have a jug with a water filter because the tap water is disgusting. We usually let it in the sink after filling it because filtering is rather slow. My sister-in-law who visited us the other day asked me if there was any technical reason why I put the filter in the sink (which is about 20 cm deep): "Is that to make it filter faster?". I tried to explain the special theory of relativity of height, but it didn't make it easier for her. She finally found peace when I explained it was just laziness to leave it in the sink. (She's not unintelligent otherwise although I must admit she often buys lottery tickets.)

Can anybody top that?

 

[DrillerNic]

Linked to ZDAS's post about Big Oil quashing the water fueled car, there's the variation that Big Oil have patented a water buring car/ electric car/ highly fuel efficient car, again to keep the invention from the world.

Of course a patent, is, by definition, a public document, and only runs for a limited amount of time....

 

[TheTick]

It always surprises me to see how many people (engineers, even) think there is a moment of "hang time" before a dropped object begins to fall. Too many "Road Runner" cartoons, I guess.

 

[bandraoi]

"Hot water freezes fater than cold water."?

Do freezers ramp up their power when the temperature goes past a certain point and work harder to bring their temperature back down?
If that was the case with some freezer designs then it might have the side effect that a glass of hot water would freeze more quickly than a glass of cold water.

 

[IRstuff]

Read the link. The phenomenon has been documented for over 2000 yrs.

TTFN

 

[dford]

It is common for people to think that vacuum does work and that "cold" flows toward warm.

Doug

 

[GregLocock]

Just an idle question - could you not explain every facet of thermodynamics by assuming that coolth exists rather than warmth?

I must confess I find temperature a fairly confusing concept at the best of times (cue IRstuff's lecture on freezing windscreens at night when the air temperatureis above freezing, and why a solar furnace can't be hotter than the sun)



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[flamby]

What is the relative speed if I move towards you and you come forward to me, both of us at 10mph? And what if the speed is that of light?

Ciao.

 

[TheTick]

Greg
I used to visualize temperature as the thermal equivalent of pressure. Heat "deflates" towards cool. To make something cooler requires a "heat pump" to create a "thermal vacuum".

A bit pedantic, but it helped me put a picture to many concepts.

 

[icelad]

Greg
It helps me to remember the ultimate villain, entropy. It takes more energy to put the gasoline together then the energy released by combustion. Thus I remember that all systems will try to move toward a less energetic state, i.e. hot will try to move to cold. (Of course, opening the refrigerator door seems to make a lie of that as you feel the cool air spill out into the warmer room)

Flamby
Similar thought. If I was standing there watching you approach at the speed of light, would you not suddenly appear and would seem that you would recede away from me as the images from farther away came to me. (The image of you at 0 feet would get there first, then 1 foot, 2 foot…100 yards, etc. Even though the farther images left sooner they could not outrace you to form an image on my eye.)

JOhn

 

[drawoh]

icelad,

Regarding glass as a liquid, check out...

http://math.ucr.edu/home/baez/physics/General/Glass/glass.html

They end up by not being sure exactly what a solid is. A lot of claims about slow flow of glass are not accurate.

JHG

 

[icelad]

Very (super) cool, It was just something I had been taught back in the early 70s
Thanks for the heads up
John

 

[swall]

Wind chill--I hear people worry about not having enough antifreeze in their radiator if the wind chill will be minus 20F even though the temperature remains above 32F.

 

[swall]

Oops. Meant to say 20F, not minus 20F.

 

[GregLocock]

Wind chill is a good one - I vaguely remember it was an experiment involving fit people standing around in the Antarctic and saying how cold they felt.

Oh we used to have a good one on the engine forum - ram air intakes. If you had a really big ram air intake funnelling down to a small throat then you'd get some huge compression wouldn't you?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rb1957]

in canada, wind chill is real (trust me) ...
but what many drivers do (particularly truck drivers) is they cover up most of the radiator ('cause they don't need the evaporative cooling)

 

[PBroad]

epoisses, try the obvious: You leave the jug in the sink as a safety measure, if it gets knocked over you don't have a wet floor!

 

[zdas04]

Wind chill is real, no doubt about it, it is the effective rate of heat transfer. When the wind is blowing hard on a day that is near (but above) freezing, your body will give up heat at a rate that is the same as the rate on a much colder day, but if you die your body will not freeze.

If the temperature is below freezing, the wind chill will accelerate the rate at which something freezes. Since it is below freezing much of the year in Canada (I've heard August through June, but that might have been an exaggeration), blocking the air flow has the effect of slowing the rate of heat transfer.

David

 

[TheTick]

Still, as far as cars and wind chill go...

In a car that is not running, the coolant will not get colder than the ambient air temperature, regardless of wind chill.

 

[GregLocock]

Sorry, that was my point. The 'calibration' of the wind chill charts you normally see were produced by Antarctic explorers standing around outside in their knickers saying "gosh, it's mighty damn chilly this morning".

As soon as you add clothing, or long term exposure, or change the fitness level or the calorie intake of the subjects, you'll get different curves.

I vaguely remember that people who live in the tropics can die of hypothermia in temperatures that we'd regard as perfectly survivable.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[hyposmurf]

1.-A bigger pipe from a elevated tank , give more pressure than a smaller one.

2.-A pump can suck far below 33 fT or 10.033 meter.

3.-Why to worry about vaccum ? , if it is only 1 Atmosphere pressure: this vessel shall resist it, an oil 205 Gl drum.

4.-If you close the output in a centrifugal pum, pressure will burst it.

And keep coming .....

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