1

Just Chat! / Re: Would it be wrong to try to get somebody kill themselves?

« on: Yesterday at 18:03:46 »

Generally considered a Bad Thing to try to persuade anyone to kill himself, but I think it is equally indefensible to bully anyone into staying alive if he genuinely wants to die. This is where the law needs a serious review. Suicide and attempted suicide are not illegal in the UK if you are sufficiently fit and compos mentis to make all the arrangements yourself. The problem arises always and only if you are so sick, disabled, distressed  or mentally incompetent that you need help: it is a crime to assist a suicide.

So suicide is statutorily denied to those who need it most. In a marginal case, where someone seeks legal assistance (as with Dignitas) it has been argued that a third party purchasing your plane tickets or driving you to the airport, is guilty of assisting a suicide. Fortunately the Crown Prosecution Service has usually ruled that such prosecution would not be in the public interest thus establishing a sensible precedent that should be embodied in statute.

2

Physics, Astronomy & Cosmology / Re: How did scientists measure the mass of the Earth and other planets?

« on: 14/10/2018 09:37:33 »

from: alancalverd on 12/10/2018 09:49:23Using F = GmM/r2 you can calculate the force on a falling object of mass m in terms of M, the mass of the earth, and G, which we assume to be a universal constant.

That doesn’t work.  We’re trying to compute at least a rough G and M here.  We don’t know either of them yet.  We do know force F is 9.8 newtons for a 1KG mass.  We can assume we know r.  We therefore know the product of G and M, but not either separately.

We don't "know" F = 9.8 N/kg. We define the newton as1 kg.m.s^-2, and have a measured value of g = F/m

We don't "assume" r: it is a measured value - see below.

As I said, Cavendish (1797) measured G directly. 

Maskeleyne estimated the density of the planet from an experiment in 1774 at Schiehallion, measuring the deflection of a plumb bob on two sides of the mountain. In fact, that was a measure of G rather than ρ_earth since the assumption that the mountain was homogeneous was more defensible. Newton had considered the possibility but rejected the experiment as too difficult.

As F = ma, we can measure the acceleration of a falling object or the period of a pendulum to get a value for F/m

F=ma works (F 9.8 = 1 (mass) * 9.8 m/sec acceleration), but that doesn’t yield either mass of Earth M nor G, which are the two things we’re trying to determine here.
The pendulum thing is a function of acceleration (9., not of the mass of Earth.  Put a pendulum in a rocket accelerating at that rate and it will have the same period as here on Earth.  It tells you nothing about the mass of the Earth under you.

You have missed the point. If we know F/m we know GM/r², and we have known r since 200 BC (Eratosthenes), so all that was required for an accurate estimate of M was Cavendish's determination of G. The reason for using a pendulum  is that we know the period T = 2π√(Lm/F) for small oscillations, which is a simpler experiment than measuring the acceleration of an object in free fall.

The most amazing fact in all this, hundreds of years and thousands of experiments later,  is that the gravitational masses of all bodies are identical to their inertial masses. Why?

Interestingly, Hipparchus estimated the radius of the moon's orbit around 130 BC, though his assumptions regarding the earth's orbit of the sun were somewhat wild.

3

Technology / Re: Are heavier-than-air hybrid airships a good idea?

« on: 14/10/2018 09:25:39 »

A disk-shaped craft is not as subject to side-winds as a slab-side , like an airship or a tube .

All aircraft are equally susceptible to crosswind in flight. You fly through a "box of air", and if the box is moving sideways or backwards in relation to your intended track, you have to go faster and change course to compensate. It's not the same as crosswind on the ground, which has the same effect as on a car or boat - you weathercock into wind depending on the shape of the vehicle and the position of its wheels or keel.

Obviously the effect is less significant at higher airspeeds, but the increase of windspeed with altitude means that even at 500 kt you need to correct for wind drift and going backwards at 150 kt and 30,000 ft is one of the joys of propeller aircraft.

4

General Science / Re: How does proton beam therapy work?

« on: 05/10/2018 18:00:46 »

Some energy is deposited all along the track, but the interaction crossection increases as the proton slows down, so a lot of energy is deposited at the end of the track. Wikipedia has a good entry under "proton bragg peak".

For a crude analogy, imagine a hot cannon ball flying through a mountain of butter. Initially, it is travelling quickly so there is very little time to transfer heat energy to each meter of butter, but when it stops moving, it can deposit all its residual heat at one point.  So the butter will re-congeal along the track but form a molten pool around the stationary missile.

5

Chemistry / Re: How to calculate the valency of 'Al' in 'Al2O3'?

« on: 05/10/2018 09:34:10 »

Even if you don't know the valency of oxygen, suppose the valency of Al is x and O is y, then you need the simplest integer solution to 2x = 3y, which is obviously x = 3, y = 2.

6

Physics, Astronomy & Cosmology / Re: Is the universe expanding faster than the speed of light?

« on: 24/09/2018 22:35:58 »

Einstein was all about relativity, not absolutism. As others have pointed out here, the energy required to accelerate a mass increases towards infinity as its speed approaches c, but speed relative to what?

If objects A and C are converging towards their midpoint B, there is no reason why they cannot both be travelling at 0.6c relative to B , but neither can see the other approaching at greater than c (with a blue shift). From which we can derive time dilatation and all the rest of that wonderful stuff that happens in colliders.

Moving swiftly from subatomic particles to the observable universe (isn't physics wonderful?) we can observe the redshift of distant galaxies to be consistent with supraluminal Doppler shift. The standard explanation is that they aren't moving away from us, but the space between us is expanding. That's where I get off this particular bus!

7

Question of the Week / Re: QotW - 18.09.24 - Why do I feel the pressure of wellies in water?

« on: 24/09/2018 17:11:07 »

The sensory system adapts quite quickly to even, constant stimuli. That is how we manage to work in a noisy environment or acclimatise to warm or cold weather. The dominant sensation of standing in a pool or bath is the temperature difference between air and water, but this sensation disappears after a few seconds. The even pressure distribution of water produces a weak sensation that is noticeable at the air/water junction but is quickly accommodated as it carries no threat or useful information.

Wellies tend to collapse unevenly producing isolated, moving,  small areas of pressure on the skin, which could be threatening (rapid current, or a snake) or otherwise important, so the system recognises the changes in pressure distribution

The same considerations apply to wearing a stocking which suddenly develops a hole.

8

Physics, Astronomy & Cosmology / Re: Is the universe expanding faster than the speed of light?

« on: 23/09/2018 23:40:49 »

Is there any reason why it shouldn't?

Here's a thought experiment. Assume that the universe U is an infinite array of roughly isotropically distributed bits of stuff. Assume that for whatever reason (and Schwarzchild gives us a perfectly good one) we can only observe a finite sphere X within that array. And assume that "gravity sucks". Now since U >> X, there is more stuff outside X than inside it, so X will expand due to the "suction" of (U - X) - i.e.the stuff outside - so the density of X will decrease, and the rate of expansion will increase.

Suppose we have reached the stage  at which points on the boundary of X are receding from us at 0.55c. Nothing in that statement to contradict relativity. But diametrically opposite points, say to the north and south of us, are moving at 1.1c relative to each other. All this means, relativisitically, is that an observer at N will not have any information about S, which is fine, but the diameter of the observable universe is increasing faster than c.

9

General Science / Re: If I wear ANY battery watch, in by a weeks time, the battery is dead. Why?

« on: 23/09/2018 23:15:58 »

The only common chemicals that attack gold at body temperature are "aqua regia", a mixture of concentrated hydrochloric and nitric acids, and mercury, which forms an amalgam used in mining and metal recycling.

If your sweat rots a gold watch, you would be wise not to travel by air - it is illegal to import aqua regia or mercury into the passenger compartment.

If anything other than aqua regia or mercury attacks your gold watch, ask the Fraud Squad or the local Trading Standards Office to visit your jeweller.

10

General Science / Re: If I wear ANY battery watch, in by a weeks time, the battery is dead. Why?

« on: 21/09/2018 23:04:34 »

If  the  cellphone charge duration is normal, then the problem lies with your wrist environment. Sweaty wrists, or failure to remove a cheap watch before washing the dishes, bathing the dog, working on a trawler, or scrubbing up, can corrode the battery contacts. Judging from your extensive  career and life experience, I find it unlikely that your grandfather had many  battery-operated watches as they did not exist before 1960.

In a spirit of scientific enquiry, you could invest in a solar-recharged watch. Accurist  and Citizen make reasonably-priced "eco-drive" watches that are well sealed against the usual insults (mine regularly cycles up to 15,000 ft and I will admit to some sweaty moments) and never need to be opened - the one I am wearing now has run continuously for over 10 years. As a control, you could wear a self-winding mechanical watch on the other wrist, and see which one fails first.

11

Physics, Astronomy & Cosmology / Re: Why can't we recycle the water in our breath?

« on: 12/09/2018 21:50:10 »

Absolutely standard procedure where it is necessary to do so artificially (spacecraft) - air conditioning and urine recycling plant conserves water as far as possible. On the surface of the earth, all the water vapor eventually finds its way into a cloud, then rain, river, and back to the tap via the usual filters: 100% natural process driven by the sun.

12

Just Chat! / Re: Hand Washing

« on: 10/09/2018 18:22:22 »

A lifetime's experience, including mechanical engineering, sterile clinical physics, commercial baking, gardening, and working as a detergent chemist,interspersed with hosing down muddy kids and dogs that like to roll in diesel puddles when there's no fox poo around.

Also studying the instructions on the tin and various health and safety posters.

13

Physics, Astronomy & Cosmology / Re: How do you steer a rocket following take-off?

« on: 07/09/2018 10:04:46 »

By building and fuelling the rocket vertically, you avoid any problems of it bending or loading asymmetrically before takeoff, and you have optimum access to  all parts of what is essentially an axially symmetric machine.You are thus constrained to a single "runway", pointing upwards.

Having left the runway in any sort of craft, the next task is point it where you want to go, which is rarely the same as the runway heading. Turning east gives you a useful 1000 mph contribution to speed in that direction, so the optimum initial heading may be a compromise between attaining orbital speed and final course.

A small asymmetry of the thrust vector will turn the rocket. This can be done with moveable vanes in the exhaust of a single motor, altering the thrust angles of multiple primary motors, or adding secondary steering motors with thrust vectors perpendicular to the primary drive.

14

Physics, Astronomy & Cosmology / Re: Are there any ways to show whether nature is fundamentally continuous/discrete?

« on: 05/09/2018 22:35:52 »

No.Spacetime is a mathematical concept used to model the observed universe, and for the most part it works best as a continuum. It's easy to plot discrete events on a continuum, but very difficult to do so on a granular surface.

15

Geek Speak / Re: Can you recommend how to protect a clinic lab from malware?

« on: 03/09/2018 10:50:05 »

Remote support always and only through the dedicated data line to the factory (essentially a point-to point dialup), never public internet. Rented data lines can be hacked, but it's a hard target for no reward: if you freeze my MRI control processor, I can just reboot the whole thing from scratch, and all the patient records and image data are on the clean system where not even the factory can input - though I can send them an image through the dirty server for fault analysis. The only people who might find the software of interest  are rival MRI manufacturers, but their primary hardware is so different that it would be quicker to write their own.

If a supplier insists on using a particular system, make sure he is contractually liable for any failure. We have encryption on the dirty system between our accounts and the bank, for instance: their choice, their liability.

As Evan says in another thread, the real problem is your staff. I did find a senior employee had plugged his personal laptop into the clean system. He is no longer with us.

Sadly, the whole system has been "updated"over the last few years and may no longer be as formal or robust since I moved on, but the principle is worth considering when starting a new venture.

16

Just Chat! / Re: Education review and impressing rights and freedoms, democracy, myth busting.

« on: 02/09/2018 16:36:24 »

Good deeds do not require supernatural justification. The only rational reason to spend time appealing to the supernatural is therefore to justify or excuse evil deeds.

17

Geek Speak / Re: Can you recommend how to protect a clinic lab from malware?

« on: 02/09/2018 16:13:04 »

I built a diagnostic imaging clinic with two physically separate networks. All patient clinical data and images circulated on one "clean" system with no direct connection to the outside world. Anything that had to leave the premises  did so through a push-only port to the "dirty" server where it could be picked up and attached to invoices, emails etc., along with any incoming medical records, scan requests, facebook, pornography and advertisements that got though the firewall.

Equipment performance monitoring and updates went through a separate processor and a dedicated data line to the factory with no internet capability.

Grumbles about the cost of "why two servers and all that hard wiring" etc when I set it up (servers cost £5000, MRI unit cost £1,000,000), and muttering about "no Amazon in the control room" (go shopping in your own time, please). No breaches or crashes in 15 years.

18

The Environment / Re: Are thunderstorms more common in the afternoon?

« on: 25/08/2018 21:18:32 »

Yes. Particularly in summer, more rain falls in mid-afternoon (thanks to convective cloud precipitation) than at other times, but the difference is less noticeable in winter when most of the rain is advective rather than convective. However it is also the case that official daytime in Cambridge lasts almost 17 hours in June and less than 8 in December, so if the rain fell continuously, it would mostly fall at night in winter.

PS. The Camelot myth having now been assigned to Tintagel, "it do rain there most of the bloody time, me dear."

19

New Theories / Re: NFEU MODEL

« on: 25/08/2018 14:57:53 »

Inspired brilliance.

This kind of meaningless drivel belongs in the catalog of an art gallery, the program notes of some egregious modern composer, or a postgraduate seminar in quantum philosophy. Don't waste it on mere seekers of wisdom.

20

General Science / Re: What are the common methods to quantify accuracy and precision?

« on: 25/08/2018 11:57:38 »

Let's play a little bit by estimating the value of π, and sort out the answers below by their accuracy and precision.
A   3.14159265358979  true as far as it goes
B   3 good enough to estimate the value of a reasonably fit tree from its widest diameter
C   3.00 an economist's forecast: misleading implication of precision
E   3.14 good enough for carpentry
F   3.15 wrongly rounded
G   3.14159265 good enough for most navigational and engineering purposes
H   4.14159265358979 dangerously wrong
I   3.14159265358979 +/-1 wrong statement of error. should be +0.5 x 10^-14/-0 because we know the true sum of any usefully convergent pi sequence exceeds the truncated value
J   3.14159265358979 +/-10 see I above
K   between 3 & 4 a politician's statement: literally true but ambiguous and potentially misleading: it is a constant
L   between 5 & 5 Trumpism. The greatest value of pi - ever! The evil fake news media want to deprive you of your right to thirty seven percent of your American heritage by insisting on a number that even the ancient Greeks knew was irrational. And now Greece is bankrupt. Pi is five! Pi is five! .....