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?

 

[25362]

When dividing approximate physical quantities expressed in whole numbers, the larger the number of digits to the right of the decimal point, the more accurate the answer.

 

[strokersix]

hyposmurf's sucking pump reminded me of one I encountered recently. A co-worker insisted that a hydraulic cylinder with the rod side open to atmosphere and piston side closed, full of oil, would hold significant tensile load. I tried to explain that the maximum load would equal one atmosphere pressure on the rod side and more than that would draw a vacuum space on the piston side. I don't think he believes it to this day.

 

[PBroad]

Perhaps the greatest physical misconception is that USA has the engineers and resources to combat a force 4 hurricane.

 

[jbel]

Perhaps the greatest physical misconception is that USA has the engineers and resources to combat a force 4 hurricane.

It's not a question of if you have them, it's how you implement their usage.

 

[TheTick]

Sometimes nature wins, and nothing can be done or implemented) about it.

 

[PBroad]

Jbel is right others less so.

The US has some excellent LCACs that would have resolved the problems in New Orleans much faster however the USS Kearsarge and USS Harper's Ferry (the mother Ships) are deployed in the Gulf. The best comment I have heard is "A man-made levee will always collapse from lack of maintenance and logical thinking".

The levees were 16-17ft but a full 4 ft lower at the points that collapsed, eroded from the bottom. Why have a rescue centre below water level? Police cars and other emergency vehicles were some of the first to be lost, and all this after an excellent "huricane pam" exercise in July 04 that predicded all the experienced problems.

Most of the recomendations made in 1993 (Galloway report)have not been implements. Many of the levees have not been repaired since 1965. 1,900 sq miles of protective coastline have been lost since 1930, and lack of dredging has resulted in the Mississippi flowing on an "aquaduct" through the city.

The U.S. Army Corps of Engineers requested $27 million for this fiscal year to pay for hurricane protection projects just around Lake Pontchartrain. The Bush administration countered with $3.9 million, and Congress eventually provided $5.7 million, according to figures provided by the office of U.S. Sen. Mary Landrieu (D-La.).

Just to increase the levees by 10 ft will require at lease three times the volume of fill currently used as the levee has a very narrow base and just one wall, no double barrier as used in military fortifications. Then there are the permitted chemical plants built below sea level that were just accidents waining to happen!!!!

 

[Qshake]

On that anti-intuitive thought of cool air rushing from the fridge as you open it; suppose it was the warm air rushing into the cooler environ that pushed the cool air out and onto your skin......that would be consistent with the heat sink theory....wouldn't it?

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[rmw]

OK, I have a question.

On modern juke boxes, (they play CD's instead of records) there is a transparent tube of colored fluid with bubbles in it, but instead of the bubbles rising, they are entering at the top of the tubes and exiting at the bottom.

Not only do the bubbles go in a direction that is counterintuitive, they get larger as they approach the bottom instead of smaller as one would expect them to do due to the increase in static head.

I have been in the presence of these machines with many engineers, most of whom are smarter than me, and no one has offered a plausable explanation of what we are seeing.

Anyone got the answer?

rmw

 

[rhodie]

CO2 plus surfactant are an oft' attributed culprit of sinking bubbles, but a slightly different effect describes the sinking bubbles in Guinness Draught:

http://www.chem.ed.ac.uk/guinness/

I've never noticed the newer jukebox bubble light traveling the wrong way... maybe a vaccum force is involved?

 

[zeusfaber]

"Bubbles" made from a clear, denser liquid - and perhaps some change to the tube geometry to create an illusion of expansion?

A.

 

[icelad]

Qshake
Is the cooler air not denser then the warm air that it displaces, which causes the flow out into the warmer room?
John

 

[rmw]

The tube geometry is consistent throughout its length (visual observation, I haven't put a micrometer on it) and the bubbles' size changes as the bubbles (spaced about 6" apart) travel from the top of the juke box to the bottom of the jukebox. Can't see how that would be an optical dillusion. But I am open to answers.

Intuitively, one would think that if the liquid was flowing from high to low, that as the pressure dropped along the length, the bubbles should grow in size. What is observed is just the opposite.

Many engineers, and some of them even good ones, have been stumped. I have yet to have one propose a solution, though.

rmw

PS: this brand of jukebox is very common in the USA, so if anyone is out enjoying a bite at an eatery that has one, go observe the phenominon.

 

[MikeHalloran]

One particular Freon used for cleaning refrigeration systems is water-white, immiscible with water, and (very dense) liquid at room temperature. A switching valve could introduce 'bubbles' of it into a flowing stream of water.

I can't explain the size change.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Suppose the tube were tapered and yet ground so as to magnify the image towards the bottom?



Cheers

Greg Locock

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

 

[Qshake]

icelad - my primitive science background would answer yes to your proposition - the cooler air is denser than the warm air displaced.

I really don't know the answer but just posed a thought. Interesting item.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[CostasV]

You can see (some of) the obsolete scientific theories at the

http://en.wikipedia.org/wiki/Category:Obsolete_scientific_theories

Costas

 

[epoisses]

The "Rain follows the plow" theory is extremely interesting in the light of another thread in the Where is Engineering going forum.......

 

[Tobalcane]

One misconception that I can think about is the misconception that shapes (2D) with the same perimeter would have the same area. This is not correct. If you have a square that is 1in X 1in the perimeter would be 4in and the area is 1in^2. If we had a circle that had a perimeter (circumference) of 4in, the area would be 1.273in^2. Same perimeter different areas, I don’t know why but that still amazes me. Even if you go to a rectangle, say a 0.5in x 1.5in the perimeter would be 4in and the area is 0.75 in^2.

Go Mechanical Engineering
Tobalcane

 

[IRstuff]

Goes along with why bubbles want to be spherical

TTFN

 

[MikeyP]

Here's a good one. I think it was Wittgenstein who came up with it. It is so simple that if you think about it analytically for just a second, you will see the answer. Try thinking about it intuitively instead.

Put a grapefruit (~5 cm radius) on the floor. Take a piece of string and wrap it around the "equator" of the grapefruit and tie it tight. Now remove the string and cut the loop. Then add EXACTLY 2 metres of string to the cut loop and re-tie it into a new, larger loop. Form this new loop into a circle and place the grapefruit at its centre. The gap between the grapefruit and the string will be big enough for you to stand in (about 30 cm or so).

Here is the clever bit. Repeat the process but instead of a grapefruit use the whole earth. Tie the string tight around the equator (assuming a flat surface), cut the loop, add 2 m, re-tie and re-centre.

How big is the gap now? Could you slide an atom through it? could you perhaps slide a human hair through it?

Highlight below for the answer

[White]The gap is still about 30 cm. The relationship between radius and circumference is a linear one (r = c/(2pi)) so adding 2 m to the circumference of any circle always increases the radius by 1/pi metres.[/White]

M

--
Dr Michael F Platten