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General Science / Re: How long will a boiled egg last?

« on: 04/07/2018 01:25:02 »

If you're in the UK, boiling an egg is, I believe, very likely to damage the cuticle which protects the egg from bacteria, the recommendation I've seen is that once you've boiled an egg, you should refrigerate it and consume it within a week. A concern would be any recontamination that happens after the egg has boiled, and some bacteria form spores that can survive boiling anyway. As the egg dries the bacteria may be sucked through egg shell through its pores and then the egg may start to spoil if left at room temperature. And if you've peeled them, all bets are off anyway- you have to refrigerate them.

If you're in America, the eggs are washed, so the cuticle is already damaged and the egg should be refrigerated anyway, even before boiling.

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Geology, Palaeontology & Archaeology / Re: Are these Colourful Mountains Fake or Real?

« on: 03/07/2018 01:33:17 »

This image is clearly digitally manipulated- China is much too far away to see these mountains otherwise.

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Physics, Astronomy & Cosmology / Re: How much is the air drag energy at 60mph?

« on: 24/06/2018 21:03:34 »

You can calculate it from the drag equation:

F = 0.5 * p * u^2 *  Cd * A

Where p is the density of the air (depends on altitude, 1.225 kg/m^3 at sea level), u is the speed in metres per second (26.8 m/s at 60 mph), Cd is coefficient of drag, and A is the cross-sectional area of the car, viewed from the front.

And the power is F * u

So you just need the frontal cross-section and the Cd factor, they are often quoted for a car.

4

Physics, Astronomy & Cosmology / Re: Why is energy observer dependent?

« on: 24/06/2018 14:32:40 »

So what happens if we apply that to  the universe as a whole ? Is it possible that every point in the Universe  would measure all the vectors of all the  momentums ** as summing to zero?(since the centre is everywhere it seems)

** just a thought experiment I guess.

Yes, there is a centre of mass frame of the universe (it's not physically special in terms of Relativity though), which minimises the kinetic energy, and would have been the motion of the Big Bang.

It's also thought that the total energy of the universe is probably zero in that reference frame-  gravitational potential energy is negative, and kinetic and mass energy is positive. They appear to cancel out to within measurable accuracy (which isn't very highly accurate, but still, it could make sense that it would be zero.) If so, it costs no energy to make a universe!

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Physics, Astronomy & Cosmology / Re: Why is energy observer dependent?

« on: 24/06/2018 03:07:05 »

The centre of mass reference frame has the property that the sum of the momentums is zero. So if the vector sum of the momentums is zero, relative to any other frame (such as that of the Earth) then you can use that other frame instead (which is normally more convenient). So if the masses are all the same and the velocities are all equal and opposite relative to the Earth, then you can just use the Earth frame instead.

The other thing that's worth knowing is that it turns out that you can convert from the Centre of Mass frame (i.e. the zero momentum frame) to any other frame really easily by adding on half the square of the relative frame speed times the total mass.

So for example if you calculate some total energy, say 1MJ, for four cars each weighing a tonne- for simplicity, let's say it's symmetrical relative to the Earth frame, and you want to calculate it instead relative to the Sun's frame, the Earth moves at ~30km/s, so you add on 0.5 * (4 * 1000) * 30000^2 = 1.8TJ  and then you just add on the original 1 MJ = ~1.8TJ (which is quite a big number- but the Earth moves very, very fast!)

FYI it's not actually terribly hard to prove this, you mostly just substitute the Vn' = Vn+Vf in the sum of kinetic energies equation above and expand it and sum it and rearrange it a bit.

6

Physics, Astronomy & Cosmology / Re: Why is energy observer dependent?

« on: 23/06/2018 23:31:40 »

There is actually a minimum energy for a system, which is the total energy measured in the centre of mass frame.

OK, you probably don't know what that means. For simplicity, let's just take kinetic energy. Kinetic energy is a square law, so a vehicle going twice as fast, has four times the energy, not just twice.

i.e. Kn = 0.5 * Mn * Vn^2

Where Kn is the kinetic energy of a body, Mn is the mass of that body and Vn is the speed of that body.

So if you add up all the Kn's you can get the total kinetic energy.

So let's say for the sake of argument you want to calculate the total kinetic energy of all the cars on Earth.

The problem is that the velocities depend on what you're measuring the speed relative to - is the centre of the Earth? The centre of the sun? The centre of the galaxy? They're all moving differently and all of these give you WILDLY different answers.

But if you pick the centre of mass of all the cars for the reference point- it turns out that that's the minimum kinetic energy. It also turns out that you can define the potential energy independent of the frame of reference that you're calculating it in, so only the kinetic energy matters much.

7

General Science / Re: Can water freeze at 38 degrees Fahrenheit?

« on: 22/06/2018 17:33:00 »

Yes, that's a frost. You commonly get it on car windscreens, unless they're parked under carports or close to houses. It happens at air temperatures ABOVE freezing!

Frost happens because the temperature of the night sky, is very, very low, far below freezing. So the pond water is radiating infrared radiation in all directions, but being warmed only from below. So if it's reasonably close to freezing anyway, it will freeze, even if the air temperature is above freezing, because the air is transparent to infrared and a reasonably good insulator. Meanwhile the thermometer will be inside your device, so won't be cooled by the sky, and will reflect the air temperature and read much higher. It's supposed to work like that. It's not a fault.

This frosting effect was historically used in places like Iraq to make ice even in the summer, in desert conditions, they would make a large thin puddle of water and wait for it to freeze, and then collect the thin sheets of ice each morning and put it into ice houses.

Scientists have managed to boost this effect to a ridiculous degree, and actually come up with materials that can cool something down well below ambient temperature, even in direct sunlight, by producing a material that reflects sunlight away, while being really good emitters in the relevant infrared bands.

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General Science / Re: Is there a scientific basis for marathon runners carb-loading?

« on: 22/06/2018 16:51:54 »

Yes, science is generally behind the idea that carbohydrate loading before endurance races give positive improvements and can help avoid 'the wall', which happens when your muscles run out of carbohydrates.

https://en.wikipedia.org/wiki/Carbohydrate_loading

Training with depleted carbohydrate stores has a far more mixed experimental evidence for beneficial results. In principle it could uprate enzymes to break down fat stores in some way, but in practice muscles tend to burn carbohydrates together with fat, and one of the results of normal training seems to be that muscles tend to better spare carbohydrates anyway.

https://www.runnersworld.com/nutrition-weight-loss/a20803222/training-with-depleted-carbohydrate-stores/

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Technology / Re: When is the electric motor used in hybrid vehicles?

« on: 16/06/2018 15:45:55 »

There's different strategies used in different cars. The Prius has a only small battery, so the engine runs quite a bit, the battery mostly captures regenerative braking energy. The Chevrolet Volt has a much bigger battery, so runs mainly just on that, and when the battery gets depleted it runs as a series-hybrid using the engine to turn a generator to top up the battery. However, there's also a mechanical linkage, between the engine and the wheels, so it can directly drive the car, together with the electric motor, when you need a burst of extra acceleration.

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Technology / Re: When is the electric motor used in hybrid vehicles?

« on: 15/06/2018 18:56:52 »

There's many different schemes:

https://en.wikipedia.org/wiki/Hybrid_vehicle#Hybrid_vehicle_power_train_configurations

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Physiology & Medicine / Re: Do mycoses eat up Vitamin D ?

« on: 14/06/2018 22:21:06 »

I would think it unlikely that a hormone chemical such as vitamin D would have any direct impact on the mycoses. However, vitamini D may have boosted your immune system or modified the levels of minerals in your body and the mycoses may have been able to use that against you in some way. I agree, that it's plausible that the mycoses may 'eat' vitamin D though, if so that sounds like it backfired on them, it would have lowered vitamin D levels and helped keep down the mycoses.

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General Science / Re: Can a super soldier serum (just like captain america) be created?

« on: 14/06/2018 22:02:17 »

Not really, but genetic engineering ("gene doping") via injections might be possible that could massively boost muscle mass.

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Technology / Re: could this clockwork supply a city with renewable energy?

« on: 14/06/2018 21:47:23 »

"peakers", are power plants that generally run only when there is a high demand,"

OK, so Dinorwig is a "peaker" because they only run it when there is (very) high demand.

That's not technically correct. They run Dinowig when there's a mismatch between production and demand, whether on-peak or off-peak.

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Physics, Astronomy & Cosmology / Re: How do you calculate the cost of launching a Satellite?

« on: 03/06/2018 23:01:12 »

SpaceX lists payload sizes and costs for their rockets. Costs are usually low few thousands of dollars per kilogram to orbit, but it depends on all kinds of things such as how high the orbit is; geosynchronous orbit costs about three times as much as low earth orbit, also the plane of the orbit you want. I forget what the maximum SpaceX payload is currently supported, maybe 50 tonnes-ish, depending on how many boosters they stick on and so forth. If you had a big wodge of money they'd be only too glad to build a much bigger rocket for you though. There's no particular maximum size, thousands of tonnes could probably be launched, but on the low side, less than a few hundred kilograms or so, the cost goes sky-high.

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Physics, Astronomy & Cosmology / Re: What are magnets made from?

« on: 23/05/2018 03:08:30 »

Magnets are made from ferromagnetic materials; there are quite a lot of them including iron, alnico compounds, neodymium etc.

Not only ferromagnetic, but also ferrimagnetic (ferrite) magnets are quite common, but not quite so strong.

https://en.wikipedia.org/wiki/Ferrimagnetism

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General Science / Re: Can I insulate window glass with aluminium foil to keep the room cool?

« on: 21/05/2018 01:44:56 »

FYI you can get semi-silvered aluminium film made for this purpose that you can apply to the inside of the glass. It's not optically perfect though and you'll get a few tiny bubbles and dust trapped under it, but it will reflect ~88% or so of the light, and you'll still be able to see out through it and get some light in the room.

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Geology, Palaeontology & Archaeology / Re: Why has no one STOPPED the Hawaii volcano as plenty of warning was had ?

« on: 21/05/2018 01:36:53 »

Yeah, it's like plate tectonics! Everyone always talks about it, but nobody ever does anything! 🤷‍♂️

🤪

18

Physiology & Medicine / Re: What is the best treatment for canker sores?

« on: 18/05/2018 04:22:47 »

Whenever I get them I put a miniscule amount of tea tree oil on it dissolved in facecream (5-10% strength). I do this once and it goes away. I think there's bacteria there that the tea tree oil kills.

n.b. tea tree oil is technically poisonous by oral routes, but it's roughly as poisonous as alcohol, so I doubt it's doing me much harm. It probably also has weak hormonal effects, so I wouldn't make a habit of it, nor would I do it if I was potentially pregnant.

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General Science / Re: Is there technology that can see through walls?

« on: 11/05/2018 22:05:07 »

In some cases you can see people through walls using microwave radar. I think some government agencies may use it in hostage situations to know where people are standing before going in, but I don't know for sure. But yeah, it depends what the walls are made of as to how well it works, and the images would be very low resolution.

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General Science / Re: How Can I Change the Mind Of A Flat-Earther ?

« on: 07/05/2018 21:10:17 »

With all these people it's always worth asking why there isn't a single picture, not one, of the edge of the Earth and watch them squirm.