Author Topic: Why a compass points North... or not...  (Read 5600 times)

captain paranoia

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Why a compass points North... or not...
« on: March 01, 2013, 03:41:31 PM »
If you've ever wondered why a compass points North, and why it doesn't always, and why 'North' moves about, you might find the BBC Horizon programme 'The Core' quite interesting.

http://www.bbc.co.uk/programmes/b0148vph

It's all about the Earth's liquid metal core, and how it moves, and how that movement causes the Earth's magnetic field, and why that field moves about.

As with all modern 'Horizon' programmes, it's a little dumbed down, but not as much as others.

There are currently four days left to watch/download it from BBC iPlayer.

Lyle Brotherton

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Re: Why a compass points North... or not...
« Reply #1 on: March 04, 2013, 07:39:54 AM »
Looks good. I spent some time dealing with the good folk at Manchester University geology department fact finding about the earth's magnetic field,  some of which I put in an article I wrote for Mountain Rescue Magazine,  I will upload this later.
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Hobbo

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Re: Why a compass points North... or not...
« Reply #2 on: March 07, 2013, 10:14:13 PM »
Stuff like this is always fascinating.  It gets me thinking about how fast the molten core is moving to generate the magnetic field.

Then I think about how fast the surface of the Earth is moving - full circle in 24 hours, circumference of 40,075km (plot the attack points for that one!), which means a surface speed of just over 1,043mph (probably).

Hold on tight...
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Callum

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Re: Why a compass points North... or not...
« Reply #3 on: March 12, 2013, 08:51:14 AM »
Look, I know that there is a logical answer but I have never found it.

When you are travelling in a train and if you throw a ball down the carriage, in the direction the train is travelling, how come it travels at whatever speed you threw it plus the speed of the train, instead of rapidly slowing down. Or if you throw it across the carriage it moves towards the back instead of straight over, anyone know?

Hobbo

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Re: Why a compass points North... or not...
« Reply #4 on: March 12, 2013, 11:55:29 AM »
There are several long-winded answers to this, but it's worth noting your observation isn't necessarily correct.

Firstly, if you throw a ball it slows down the moment it leaves your hand - the accelerating force is no longer applied to it, so the remaining laws of physics are left to their own devices causing the ball to change direction and apparent speed: Gravity will cause a change in it's direction, the air the ball is moving in will cause drag which makes it slow down (I will ignore gradients and wind speed for the moment).

It's key to observe there are 2 relative speeds and 1 absolute speed. The absolute speed is a stationary object, which effectively has a zero-energy state. There is no energy in it to be transferred to another form (such as movement in a direction), therefore no potential energy - everything in the universe moves to a zero energy state, bleeding off it's energy as movement, light or heat. In the example of the train, the 2 relative speeds are the speed outside the train and the speed of the train itself. For the sake of argument, the train is travelling at 30mph in a northerly direction relative to the speed of the planet's surface, which we will say is 0mph.

The contents of the train are also moving at 30mph, which includes the air in the carriage (in my experience of trains, this is always stale - a fact that has no relevance in this case). So if you threw the ball in the direction of the train's movement at let's say 6mph, the speed of the ball would be 36mph.  The ball will start slowing down almost immediately (wind resistence and friction with objects in the carriage - i.e. bouncing) to the point it comes to a rest.  Assuming it doesn't leave the carriage, it will still have the force of the train applied to it, so it's speed is 30mph. Relative to the contents of the carriage, though, this is 0mph.  If it left the carriage, it's speed would be 0mph relative to the planet's surface, or 30mph in the other direction relative to the train.

What if you turned round and threw the ball in the opposite direction at 6mph? The moment it leaves your hand it is travelling at 24mph relative to the planet's surface. However, because all the forces (with the exception of gravity) applying to it are within the carriage it will start to speed up relative to the planet's surface from the moment it leaves your hand. Relative to the carriage contents it will slow down, though. So it's resting speed relative to the planet's surface is 30mph - higher than when you threw it.

Hope that helps - it may have a faint aroma of bovine turds, but that's the simplest I can make it other than saying "because it just does".

Interestingly, the same thing doesn't happen with regards to light, which remains unaffected by things like wind resistence and is only subject to a change in direction courtesy of gravity. So if you shone a torch on the train in the direction of it's movement, the speed of the light photons/waveforms won't be the speed of light + 30mph, it will always be the speed of light (Universal speed limit, as defined by Einstein).
« Last Edit: March 12, 2013, 12:06:23 PM by Hobbo »
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Lyle Brotherton

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Re: Why a compass points North... or not...
« Reply #5 on: March 12, 2013, 12:58:42 PM »
Brilliant answer Hobbo and a question I have been too scared to ask myself Cal ;)

Back in the 90's I was given a copy of Einstein's Mirror, by Tony Hey and Patrick Walters, and read it whilst based at some godforsaken place in Germany. Probably somewhat out of date now, but a brilliant introduction to relativity.
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Pete McK

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Re: Why a compass points North... or not...
« Reply #6 on: March 14, 2013, 12:42:51 PM »
Still trying to get my head around this  ::) because I was thinking about a helicopter hovering, is it stationary or is it actually flying at 1,043mph just to stand still?

captain paranoia

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Re: Why a compass points North... or not...
« Reply #7 on: March 14, 2013, 01:12:52 PM »
Well, not only is the Earth spinning on its axis, it's also rotating in its orbit around the Sun (so moving at about 67,000mph).  Which, in turn, is rotating in our local galaxy, which, in turn, is moving through the expanding universe...

Motion; it's all relative...

Hobbo

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Re: Why a compass points North... or not...
« Reply #8 on: March 14, 2013, 01:38:54 PM »
I am mentally rolling my sleeves up to answer this.

Translating your helicopter analogy into the ball in the train analogy, the helicopter is the ball and the world is the train (I admit, this statement makes me want to hug a tree or something). I'm going to digress, but it will come back round in the end, don't worry (hopefully this will not be a load of rubbish!)...

There is an inevitable, bigger question to be asked which I will try and answer with my "pig physics" understanding. The question being, "If the surface of the planet moves at 1,043mph why do we have 5mph winds?" I think the best answer for that is "google wind gradient". In simple terms, the friction caused by the surface of the planet as it rotates stops the wind lower down from zipping round as it saw fit and making old ladies end up in trees due to high speed. Ultimately the friction of the surface of the planet moving through the air causes the air molecules to move with it. So close to the ground you would have slow wind speeds, and higher up you would have higher wind speeds.  I appreciate there are weather patterns and air convection to consider (the air being heated up will rise relative to cooler air around it), but hopefully you get the point. This same effect is why I'm able to use a spoon to stir my tea and make it a proper strength (instead of the horrible watery milk my wife likes). So that's why the air moves relative to the planet. It doesn't bunch up at the back because the earth's movement is consistent - if it suddenly stopped in space...I think we all know what would happen and we don't need a Michael Bay SFX session to imagine it.

This brings me back to the helicopter.  We have established that the air moves at the same speed as the surface of the planet (or relative to it). The helicopter sits within the air being affected by the wind speed and the earth's gravity. Because the wind is moving so slowly (relative to the earth, of course!) then the helicopter will sit there quite happily. If, for some bizarre reason, the air speed had a sudden gust up to 1,043mph, not only would we be picking old ladies out of trees for months, but the helicopter would not hover.

So to answer you're question, the helicopter is staying where it is, the blades are generating sufficient lift to keep it at 1 altitude (courtesy of gravity and air pressure) and the wind holds it in place. This is ignoring the fact that a helicopter never truely hovers - the pilot has to make constant adjustments to keep the helicopter in 1 place (though some newer helicopters have features that make this easier. Apparently.).

Hope that helps - it's a very simplified view of it all. If someone has a better answer than that (I'm sure there are lots of better answers than that) please shout up! And a massive pat on the back for me talking about flying without saying how British pilots are the best  8)

Stay tuned for next week's science lesson - I'll be dissecting a horse's eyeball (the rest of the horse went in the lasagne).
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Hobbo

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Re: Why a compass points North... or not...
« Reply #9 on: March 14, 2013, 01:39:59 PM »
It is all relative, CP, but with laws of physics thrown in.
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Lyle Brotherton

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Re: Why a compass points North... or not...
« Reply #10 on: March 14, 2013, 02:30:19 PM »
Not all helicopters hover!

After SARSCENE 2008, held in St Johns, Newfoundland, I decided to stay over a few more days and visit some of the rescue teams I had met at the conference plus take in some of the dramatic scenery of the island (evidently it was once part of northern Scotland).

I was particulariy excited at being asked to fly up the Labrador Straights in the new that Sikorsky S-92 SAR helicopter, I hoped to catch a glance of some Minkes, White-Sided Dolphins, Beluga or my favourite sea mammals, Killer Whales and of course the ubiquitous icebergs, this area is one of the best places to see them in the world.

We did not have long to fly before we came across a massive aquamarine berg, as large as some small Scottish islands, and the guy’s suggested that I might like to grab some ice and take it back to drink with the famous Canadian vodka ‘Platinka’.

The pilot descended to just a few centimetres above the berg and I jumped out, at which point the helicopter departed. My initial humour started to slip into the irrational, was this iceberg going to turn turtle, was this a plot – it was a long 10 minutes or so and caused much laughter, which when my feet were back on terra firma at Bridie Molloy’s bar in St John’s, I joined in with and where I met the great Ted Bates – but that is another story :)
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captain paranoia

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Re: Why a compass points North... or not...
« Reply #11 on: March 14, 2013, 06:44:52 PM »
> It is all relative, CP, but with laws of physics thrown in.

Yes.  Primarily:

http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion

In particular, the first one.

Plus some fluid dynamics for the sticky atmosphere, including turbulent flows, boundary layers, various gas laws regarding temperature and pressure, and black body radiation, etc.

Let's not get into the question of whether a plane on a conveyor belt could take off...

Hobbo

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Re: Why a compass points North... or not...
« Reply #12 on: March 15, 2013, 09:28:51 AM »
I like this.

I'm gladly passing the physics baton to you  :)
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Barry G

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Re: Why a compass points North... or not...
« Reply #13 on: March 17, 2013, 03:58:49 PM »
There are several long-winded answers to this, but it's worth noting your observation isn't necessarily correct.

Firstly, if you throw a ball it slows down the moment it leaves your hand - the accelerating force is no longer applied to it, so the remaining laws of physics are left to their own devices causing the ball to change direction and apparent speed: Gravity will cause a change in it's direction, the air the ball is moving in will cause drag which makes it slow down (I will ignore gradients and wind speed for the moment).

It's key to observe there are 2 relative speeds and 1 absolute speed. The absolute speed is a stationary object, which effectively has a zero-energy state. There is no energy in it to be transferred to another form (such as movement in a direction), therefore no potential energy - everything in the universe moves to a zero energy state, bleeding off it's energy as movement, light or heat. In the example of the train, the 2 relative speeds are the speed outside the train and the speed of the train itself. For the sake of argument, the train is travelling at 30mph in a northerly direction relative to the speed of the planet's surface, which we will say is 0mph.

The contents of the train are also moving at 30mph, which includes the air in the carriage (in my experience of trains, this is always stale - a fact that has no relevance in this case). So if you threw the ball in the direction of the train's movement at let's say 6mph, the speed of the ball would be 36mph.  The ball will start slowing down almost immediately (wind resistence and friction with objects in the carriage - i.e. bouncing) to the point it comes to a rest.  Assuming it doesn't leave the carriage, it will still have the force of the train applied to it, so it's speed is 30mph. Relative to the contents of the carriage, though, this is 0mph.  If it left the carriage, it's speed would be 0mph relative to the planet's surface, or 30mph in the other direction relative to the train.

What if you turned round and threw the ball in the opposite direction at 6mph? The moment it leaves your hand it is travelling at 24mph relative to the planet's surface. However, because all the forces (with the exception of gravity) applying to it are within the carriage it will start to speed up relative to the planet's surface from the moment it leaves your hand. Relative to the carriage contents it will slow down, though. So it's resting speed relative to the planet's surface is 30mph - higher than when you threw it.

Hope that helps - it may have a faint aroma of bovine turds, but that's the simplest I can make it other than saying "because it just does".

Interestingly, the same thing doesn't happen with regards to light, which remains unaffected by things like wind resistence and is only subject to a change in direction courtesy of gravity. So if you shone a torch on the train in the direction of it's movement, the speed of the light photons/waveforms won't be the speed of light + 30mph, it will always be the speed of light (Universal speed limit, as defined by Einstein).


Even in a weightless environment like space without gravity, these principals apply. Astronauts while taking a space walk outside the space vehicle maintain the same speed as the vehicle which is over 17,000 mph.
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