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Physics, Astronomy & Cosmology / Re: What is the effect of MRI on body?

« on: Today at 13:14:32 »

The effective dose from a single intraoral x-ray is around 10 microsievert. This is equivalent to about a days' natural background radiation and the risk of inducing a fatal cancer is less than 1 in 1,000,000, officially described as "negligible"

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Physics, Astronomy & Cosmology / Re: Is all motion relative?

« on: Yesterday at 20:26:23 »

If A accelerates, she can measure this acceleration, thus, she knows she is moving; although there is nothing relative to which she can be said to be moving.  This begins to look like “absolute” motion; but can have little, or no, real meaning.

You have put your finger on the nub of newtonian mechanics. Quite simply, motion is a change in position vector, but in the absence of an origin for that vector, there is no way that a body can "know" it is moving at a constant speed. Beware of getting philosophical and introducing terms like "real meaning" - it can poison your physics!

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Just Chat! / Re: How effective was the "berlin wall"

« on: 16/02/2019 12:32:22 »

Walls are primarily of propaganda value, and their construction can be a means of the state covertly financing manual labor in times of high unemployment.

Trump's fanciful election boast that the Mexicans would pay for it was efficiently demolished by a television interview in which President Nieto briefly switched to English with "I no pay your f***ing wall" before reverting to important matters of state. And thus it came to pass.

In the words of General Patton: "Static defenses are monuments to human stupidity."  Drugs, and the serious criminals associated with drugs, travel by sea and air.

Apropos the Berlin wall, various friends and acquaintances over the years absconded from ships' or aircraft crews or organised tours, timed a sprint across no-man's land, or in one case "borrowed" a herd of cows to walk through a minefield. As with all genuine refugees, they were a cut above average for intelligence and risk assessment, and all ended up as very successful consultants or entrepreneurs. My favorite, who sadly I never met, worked in an umbrella factory. He made a black hot-air balloon from stolen umbrella fabric and bits of gas stove, and flew quietly (using continuous marginal burn) over the border one night.

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Technology / Re: Do HV powerlines cause a hum on aircraft VHF communications

« on: 15/02/2019 16:52:53 »

If I'm ever below the power lines, I'm not concentrating on the radio. We certainly get occasional ionospheric effects but they tend to be amusing longrange skips rather than annoying interference.

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Physiology & Medicine / Re: Do flight attendants get hearing loss?

« on: 15/02/2019 10:25:52 »

It's surprisingly difficult to get any figures for cabin noise, but 60 - 70 dB seems average for a modern airliner in cruise. 80 dB is an action level for noise at work and it is normally only exceeded at takeoff or with rowdy passengers. It's a "busy" rather than "noisy" work environment these days.

The cockpit of a jet is much quieter than the cabin: airline pilots' headphones are generally of lighter construction than those of piston-engine and helicopter jockeys. Noise-cancelling phones are OK when they work, but most rely on batteries that fail at critical moments - just a couple of days ago I had to lend my old "student" passive headset to a corporate jet pilot for exactly that reason!

Interestingly, executive jets now have optional white noise in the cabin to help the passengers sleep. I guess it's safer than whisky.

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Physics, Astronomy & Cosmology / Re: How do we measure the energy of a photon?

« on: 14/02/2019 16:37:30 »

so, a photon measured in energy in a Gamma spectrum does not have the same energy as a photon measured in a AM frequency.  yet a photon is a photon. it's malleability is wave intensity as I asserted.   

A pigmy shrew does not have the same mass as a whale. So what? A mammal is a mammal.

If "wave intensity" means anything, it might just refer to the root mean square of its amplitude, which isn't at all relevant to the energy of a photon.

It is important, in any discussion, to use a common language. The language of physics is very precise.  Merely assembling sciency words into what looks like a sentence with a question mark, won't improve your understanding: if you don't understand the question, you certainly won't understand the answer.

In response to the meaningful question, I have no doubt that there are ever more accurate measurements of everything that has ever been measured. That is why we are moving to redefinitions of all primary standards in terms of universal constants rather than lumps of metal, but it doesn't alter the fact that the energy of a photon is measurable in priniciple and adequately measurable in practice.

Those of a philosophical bent may indeed ask "what do you mean by measure?" but the straight answer is always "compare x with y where y is an agreed standard".

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Physics, Astronomy & Cosmology / Re: How much wind to blow over a truck?

« on: 13/02/2019 23:59:53 »

The wind force F on a flat surface is ½ρv²A where
ρ = air density
v = wind speed
A = surface area.

To tip a truck over, you need to move its center of gravity outside the wheelbase.

Consider the wind force F to act through the midpoint of the side of the truck. This gives a turning moment of hF/2 where h is the height of the truck. The restoring moment is Ww/2 where W is the weight of the truck and w is the wheelbase width. If hF> Ww, the windward wheel will lift.

Counterintuitively, it may be easier to tip a loaded truck than an empty one, because adding load raises the CG. If W is constant, you need the same force F to lift the windward wheel off the ground, but with a higher CG you don't have to lift it so far in order to tip the vehicle over.

In practice, things get more complicated. A gust can  cause an articulated truck to snake, and the resultant centrifugal force can tip it even though the initial gust couldn't.

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Physics, Astronomy & Cosmology / Re: Can we detect flow of radioactivity?

« on: 13/02/2019 23:53:29 »

I hereby disassociate myself from any semblance of concurrence, for fear of reputational contamination.

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Physics, Astronomy & Cosmology / Re: How do we measure the energy of a photon?

« on: 13/02/2019 23:49:41 »

Having spent many happy hours measuring the energy of photons by various means over many years, I find it surprising to be told that it can't be done.

But then anyone who thinks "waves of intensity" belongs in a scientific discussion has clearly been reading too many Mills & Boone bodice-rippers and not enough elementary physics texts.

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Physics, Astronomy & Cosmology / Re: What are substances or reactions that generate UV rays?

« on: 12/02/2019 22:20:47 »

You can get battery-powered UV lamps. "Ultraviolet counterfeit detector" is available from Amazon for about £10, and there are small mains powered UV inspection lamps in engineers' tool catalogs.

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General Science / Re: What determines the directions of flights?

« on: 12/02/2019 15:03:38 »

All true, plus, if you bend the flight path slightly eastwards you can get a 200 knot tailwind from JFK to PEK.

Additionally, magnetic north is about 10° west of the US east coast, so to fly true north from New York you actually fly about 10° east on your compass. All flight directions are given as "magnetic" so having reached cruising altitude you will be initially instructed to "fly QDR 010" to stay in your airway.

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New Theories / Re: Is my Theory Of Gravitational Field valid?

« on: 12/02/2019 14:54:17 »

It is my duty to spread my theory.

You have a more important duty of intellectual honesty, to yourself at least.

Writing down equations that obviously don't make sense, and stating that a continuous function like cr goes to zero at large r, won't fool anyone into believing that you know what you are talking about.

c. 1599-1601, William Shakespeare, Hamlet, Act I, Scene III: “This above all: to thine own self be true. And it must follow, as the night the day. Thou canst not then be false to any man/Farewell, my blessing season this in thee!”.

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Physics, Astronomy & Cosmology / Re: What are substances or reactions that generate UV rays?

« on: 11/02/2019 22:41:32 »

The most common source of controllable ultraviolet is a mercury vapor discharge lamp. Looks just like a fluorescent tube but without the fluorescent inner coating, and preferably with a quartz envelope. Cheap and very effective.

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New Theories / Re: Is my Theory Of Gravitational Field valid?

« on: 11/02/2019 08:51:28 »

So according to your definitions, GE = MA/g = MAr²/GM = Ar²/G = c²r²/G²

This is unfortunate as it makes the gravitational energy of an object independent of its mass. The idea that any property of an object (apart from its color) is independent of  its mass, does rather clash with most of physics.

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Geek Speak / Re: Is windows 10-18334 the answer

« on: 10/02/2019 22:15:20 »

It's better with rice noodles and a dash of soy sauce. 

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That CAN'T be true! / Healthy Eating or Dodgy Science?

« on: 10/02/2019 17:27:47 »

Just copied this from an apparently authoritative on-line source

Gouda cheese has 819 mg of sodium per gram, and American cheese has 1,671 mg of sodium per gram. That’s a lot! Next time, make sure to cut down on the amount. It’s not bad if you cut it out completely, but this would be a sad day indeed.

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Physiology & Medicine / Re: Can a nuked thyroid gland regenerate?

« on: 09/02/2019 18:58:07 »

The rock musician's motto: "the older we get, the better we were."

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Physiology & Medicine / Re: Can a nuked thyroid gland regenerate?

« on: 09/02/2019 00:03:30 »

Total ablation of the thyroid is not always required or achieved. It is entirely normal to review all "lifetime" medications from time to time to better approximate normality.

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General Science / Re: Should I worry about electromagnetic fields (EMFs)?

« on: 08/02/2019 23:49:23 »

....which is why we use line conditioners and uninterruptible power supplies for all life-critical systems.

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General Science / Re: Should I worry about electromagnetic fields (EMFs)?

« on: 08/02/2019 17:27:22 »

...or from coal, Russian gas, or Saudi oil. Today is a good day to boil a kettle with wind power, but don't try to heat your house with it: the coldest days are windless in the UK!

Seriously, none of the sources you mention has any demonstrated physical health detriment. Tablets and mobile phones will destroy your sanity and ruin your children's lives, but not through physics or chemistry.