41

The Environment / Re: How Is A Tidal Movement In A Lake Formed ?

« on: 30/07/2010 16:15:25 »

Apparently a seiche isn't necessarily tidal - it is a resonance in a lake. A bit like if you make waves in a bath at the right speed those waves get larger and larger until the floor gets wet, and you get shouted at.

According to wikipedia
http://en.wikipedia.org/wiki/Seiche

They can be driven by wind, or small earthquakes, They will have a much longer period than other waves in a lake, and so in a big lake they may appear like tides. Apparently in Lake Eire they can get up to 5m high with a period of about 16 hours.

42

Science Experiments / Where can I see some really cool science shows and experiments in the UK?

« on: 30/07/2010 13:24:53 »

Cambridge Science Festival is very child friendly there are a huge number of things happening on the two main saturdays, which are designed around children

The only issue with the Cambridge Science Festival is that it is somewhat successful. I am involved with an event called Crash Bang Squelch (http://www.srcf.ucam.org/chaos/cbs) which is full from about 20mins after it opens at 10am, and we start issuing timed entry tickets, but I think they were all gone by about 11:30 this year, so get there early...

The queues for the Naked Scientists talk were scary!!

43

General Science / Would a walking machine work with three legs?

« on: 30/07/2010 09:59:28 »

But on that argument humans should need big heavy gyroscopes, a three legged gait is not intrinsically stable, so you would need complex control algorithms but probably not as difficult as a 2 legged one.

44

Science Experiments / Flame Tornado - A Spinning Column of Fire - Garage Science

« on: 30/07/2010 09:10:07 »

Centrifugal force? Really?
If you're actually going to explain things, don't throw false notions into the mix.
Inertia exists, centrifugal forces do not. They are only a manifestation of inertia in the local reference frame, as such claiming that "they" are doing this or that is highly misleading. Nothing is acting on the mass to throw it out or its orbit.

It doesn't exist in a stationary reference frame, but that doesn't mean that it isn't a useful concept. Centrifugal force is perfectly reasonable to use if you are in a rotating reference frame. It is also a useful intuitive shorthand for 'the air is moving rapidly so in order to make it go around a corner you have to apply a large pressure difference' and so I think educationally useful.

45

Science Experiments / Flame Tornado - A Spinning Column of Fire - Garage Science

« on: 30/07/2010 09:06:43 »

The flame is fueled using fire/BBQ lighting gel, but any hydrocarbon based fuel that burns with a yellow flame will work well

46

Science Experiments / Where can I see some really cool science shows and experiments in the UK?

« on: 29/07/2010 16:37:07 »

Cambridge science festival is great,
http://www.admin.cam.ac.uk/sciencefestival/

Pete Wothers lectures are incredible in the amount of work that has gone into them - not that I am jealous..

I also do various lectures as part of the festival both for the naked scientists and CHaOS (Cambridge Hands on Sceince)

If you want something a bit sooner have a look at other science festivals, or science centres. I haven't seen any of their shows but the centre for life is very well run, and they keep winning the best demo competition at the British Intereactive Group Event (not quite conference).

Is that any use?

47

General Science / Which mains system is safer?

« on: 29/07/2010 16:25:55 »

So, now we are down to the number of poles switched. Outlets in the UK are switched to reduce the risk of electrocution. Outlets in the US are not switched because the risk of electrocution is reduced by virtue of the voltages (to ground) that appear at any outlet.

Is it just to reduce the risk of electrocution? It also has advantages in being able to switch things off and know a fault isn't going to produce a fire - reduce transformer leakage etc.

Of course the big advantage of 230V is that you need half as much copper to carry the same amount of power..

48

Technology / How Much Does It Actually Cost The Companies To Send A Text ?

« on: 29/07/2010 15:03:02 »

The issue with a telecoms provider is that they build a big system of pipes which is really expensive, but they have very low running costs. So if you already have the system then texting is really cheap to implement, but if you had to build a mobile mast infrastructure for it, then it would be stupidly expensive. A text really doesn't use much of the available bandwidth, so in one sense it is cheap, but then again it wouldn't work without several billion quid's worth of kit already set up..

So they charge what people are willing to pay, as what a text costs the telco is very hard to work out - the big costs being the towers and subsidising the phone sales, neither of which scale with the number of texts you use.. or even very strongly with the number of calls you make.

49

Science Experiments / Dave, how did your laser experiment work?

« on: 14/07/2010 18:19:05 »

Yes it certainly wasn't working well, some of the shots seemed to produce a bright spot rather than a line which you would expect from a spark or a diffuse source. The stimulated emmission means that light that passes along the chamber is amplified so forming a spot.

 I had great difficulties getting it to even work this well, I think because I couldn't get the shape of the plates right.

50

Physics, Astronomy & Cosmology / How much energy is required to continualy orbit the Earth in less than 84 minute

« on: 12/07/2010 11:31:53 »

Just to make it more complex, if the energy or reaction mass supply for puck's rocket are carried on him, his mass will change as he orbits. Even with photon drives, his mass is going to decrease as it fires a bit. So you have to solve a fun differential equation.

Essentially
a = F/m

F= (g Isp /m)  dm/dt

m = mass of fuel
a = acceleration required = 33.45 m/s²
Isp = figure of merit for your rocket engine for shuttle = 450s
g = earth's gravitational field (from defintition of ISP) = 9.81

if you integrate those up you get
m = e^at/gIsp

so if he has to orbit for 40mins = 2400s
m = e^18.2
  = 80 000 000kg of rocket per kg of puck

As this is hydrogen oxygen fuel the energy is going to be silly at anout 285kJ/mol of water formed this will use of the order of 1 x 10¹⁵J /kg of puck

You would be better off using something with a higher Isp like an ion engine using an ion thruster with an Isp of around 3000 you would only need 15kg of fuel per kg of puck. The energy required will be the integral of the force times about 40kW/N

so I get about 340MJ/kg

which may or may not be right according to my ability to integrate.

51

The Environment / Re: Could the thin air in Jo'burg affect the flight of a football?

« on: 23/06/2010 17:51:40 »

One way of approaching this is from an aircraft perspective. Aircraft instruments measure something called indicated airspeed, this is the same as the true speed through the air at ground level and a certain pressure and temperature.

A rule of thumb is that the IAS under-reads by about 2% per 1000ft of altitude. The highest stadium is in Polokwane at about 4000ft, so the IAS would be 8% wrong. So a football will experience drag as if it was going about 8% slower than it is actually moving.

What makes it more complex is that the drag force on a football drops radically over a certain speed (the air becomes turbulent which acts like dimples on a golf ball making the ball fly further). Footballers take advantage of this effect to confuse goalkeepers.

http://physicsworld.com/cws/article/print/1533

So at ground level if you can kick the ball at about 25m/s it will go a lot further than if you kick it at 20m/s, however in a high stadium this transition will occur at an 8% higher speed. If you can only just kick the ball fast enough normally, instead of the ball moving in the low drag regime it could well be in a high drag one, and not go nearly as far, or slow down more than you were expecting.

The transition between the two types of flow is also used in curving shots to confuse the goalkeeper - if a curving shot suddenly moves into the high drag regime near the goaley it will suddenly both change speed, and if it is spinning start swerving radically, - making it very hard to catch. However if it never got into the low drag regime it won't go through the transition and will be much easier to catch.

There is of course also an effect on the players which may well be larger, but it sounds plausible that a ball could be moved a few 10s of cm by the pressure difference which when shooting into the corner of the net may make all the difference.

52

General Science / How Many Mega-pixels Is My Eye ?

« on: 23/06/2010 09:38:11 »

Something which has been hinted at so far, but I thought I would say explicitly:

One big difference is that a camera has a constant pixel density all over the image, where as your eye has a very high pixel density at the fovea and a much lower one everywhere else. So you see 'what you are looking at' at a much higher resolution (and in better colour) than everything else.

This actually makes the job of making a camera more difficult as when looking at a photo you can move your eyes around, so if the camera tried the same trick as your eyes, you would complain that the edges of the photo looked pixelated or at least out of focus. So the camera manufacturers need to pump out high resolution everywhere with a consequent increase in data that they have to throw around.

53

Science Experiments / Rubens' Tube - waves of fire - Garage Science

« on: 21/06/2010 09:17:55 »

What would the optimum diameter for the tube be ? As this could cause a problem if copper were to be used and what distance did you put between your holes ? Thanks

I used 1mm holes at a 5mm spacing. I am not sure of optimum diameter. I think for a 1800mm tube like mine more the 50mm diameter is great for higher order modes, but the fundamental doesn't work quite so well. I also had a test tube which was only 20cm long which would only do fundamental and first harmonic, I think because the cross section was larger than the second harmonic, so complicated 3D standing waves started happening.

54

Science Experiments / Rubens' Tube - waves of fire - Garage Science

« on: 21/06/2010 09:15:00 »

I am in the process of making a rubens tube for my physics class and having some difficulty.  After lighting the tube I will only recieve a small blue flame instead of a larger orange flame.  Also it won;t stay lit.  It will go out almost immediately after lighting.  Any help or suggestions would be much appreciated.

Sounds like you are not waiting for long enough, or just don't have enough gas flow, so the flames are very oxygenated. I don't know what kind of gas you are using, methane would probably burn cleaner than propane anyway.

55

Chemistry / Why did lead nitrate and copper sulphate precipitate with sodium hydroxide?

« on: 18/06/2010 12:23:58 »

In an ionic solution you don't have CuSO₄ and Pb(NO3)₂ you have Cu²⁺, SO₄^2-, Pb²⁺ and NO₃^- ions floating around freely. If you add a load of OH^- ions these will also float around.

However hydroxides are generally not as soluble as sulphates so depending on the concentration of OH the lead and copper ions will form a precipitate with the hydroxide ions. Which drops out first and in what proportion will depend on their relative solubilities.

56

General Science / Wind powered plane?

« on: 18/06/2010 11:14:48 »

Yes essentially wind shear or wind-ground shear is where you can get energy from. The fastest model aircraft is I think a glider where the record sits at getting on for 400mph. They work in an area of high wind shear looping between the slow and fast moving air.

With high lift to drag ratios you can go a lot faster than the speed difference of the fluids

57

General Science / Can eating tuna and other fatty fish help to prevent memory loss?

« on: 15/06/2010 14:47:34 »

Ben Goldacre (www.badscience.net) has a lot to say on this subject. And whilst there has been a lot of biased research done by this $1bn industry, it would appear that there is very little decent research which shows much of an effect.

58

Just Chat! / Re: Could BP blow up the oil well to stop the leak?

« on: 10/06/2010 10:35:24 »

Yes there seems to be a lot of politics being played with this. As far as I can tell, a combination of Transocean, BP and the US government has screwed up at the beginning, but since then BP has been doing a hell of a lot to try to fix the problem. Within 2 days they had effectively spent a third of a billion dollars on drilling the relief wells. The fact that noone in the industry had come up with a solution to this failure mode this deep, is a system wide problem, not just a BP one.

Unless the US government retrospectively passes lots of laws to deliberately take out BP in some kind of bizzarre politically motivated revenge, it won't kill the company. It is just too big. It would also take out a large section of the UK economy, which is struggling somewhat as it is, which would have political ramifications.

59

Geology, Palaeontology & Archaeology / Re: Why don't BP use a shape-memory metal to stop the oil leak in the Gulf?

« on: 10/06/2010 09:18:14 »

My guess is that doing anything down there is very very difficult, and that getting anything in there which is a good enough fit to be useful would be very difficult. Particularly considering that you are trying to thread a hose pipe into a slightly larger hose pipe which is on full power from a mile away. If you insert something over something else you can use a funnel to make the alignment easier, putting something inside is more difficult.

Your suggestion makes perfect hydraulic sense, but the engineers trying to fix the problem will know this too, so there must be some good reason...

60

Physics, Astronomy & Cosmology / Why does an atom not collapse?

« on: 07/06/2010 16:54:00 »

An electron is a wave/particle. The probability of where the electron can be found is defined by its wave function. If the atom was 2D you could imagine a circular piece of string around the atom with wave travelling on it, if the wavelength is a whole fraction of the circumference a standing wave will build up. This has lots of amplitude, so you are quite likely to find an electron there it is an allowed state. Other places and the wave will cancel itself out -> no amplitude -> no electron.

The minimum distance this will happen at is when the circumference is 1 wavelength this would be the lowest state the electron can be in, it couldn't get any closer.

A 3D atom is a lot harder to think about, but the allowed states are there for a very similar reason.