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  #741  
Old 12.06.2009, 13:24
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Re: Ask a Scientist

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I don't know... I agree that the gravitational force is different but the weight is the same:

Force=m(in kg) x g (9.81 m/s^2)

The radius of the Earth etc is accounted for in g, not in m ! So m is the same for both (1 kg)

isn't knitpicking fun
You're right that m is the same for both. m is mass though; F (the gravitational force) is weight.

So a different radius of the earth* means a different g, and a different g means different weight.

* more precisely, a different distance from your centroid to the earth's centroid. We usually take this to be the "radius of the earth", but that's because we're usually standing on the surface of the earth! The difference in g between sea level and Mt. Everest is about 0.3%.
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  #742  
Old 12.06.2009, 13:27
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Re: Ask a Scientist

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.
How did they make the non-stick stick to my frying pan?
.
They scratched up the pan first, sprayed it on mixed with glue, baked it on to set the glue, and then sprayed more of it on, without the glue.

Do that with scrambled eggs and you won't get them off either.
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  #743  
Old 12.06.2009, 13:34
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Re: Ask a Scientist

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You're right that m is the same for both. m is mass though; F (the gravitational force) is weight.

So a different radius of the earth* means a different g, and a different g means different weight.

* more precisely, a different distance from your centroid to the earth's centroid. We usually take this to be the "radius of the earth", but that's because we're usually standing on the surface of the earth! The difference in g between sea level and Mt. Everest is about 0.3%.
Sooo, by the time I reach Mt everest on my bicycle it will get easier to cycle as g gets lower
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  #744  
Old 12.06.2009, 13:34
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Re: Ask a Scientist

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Nope. We don't say a helium balloon is weightless or has negative weight, after all - we say it's lighter than air. It still has the same weight, it's just that the equivalent volume of air has more weight, so it displaces the balloon.

The only way air affects weight (as defined above, the net of all gravitational forces) is by all the little air molecules overhead exerting their own gravitational pull. There would be more air molecules above the iron than above the cotton, resulting in a slightly stronger gravitational force upward.

Enough to counteract the increased gravitational attraction from being closer to the earth though? Not hardly! The basic reasoning here is that the earth's gravitational pull is already dominant over the atmosphere's (otherwise you'd fall up, right?) and so moving toward the earth increases that, more than it decreases the atmosphere's.
Archemides principle says:
This principle states that a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid it displaces.Using this principle, it would have been possible to compare the density of the golden crown to that of solid gold by balancing the crown on a scale with a gold reference sample, then immersing the apparatus in water. If the crown was less dense than gold, it would displace more water due to its larger volume, and thus experience a greater buoyant force than the reference sample. This difference in buoyancy would cause the scale to tip accordingly.

So doesn't it mean that the coton has become lighter on air, to the amount of weight of same volume air?
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  #745  
Old 12.06.2009, 13:54
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Re: Ask a Scientist

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They scratched up the pan first, sprayed it on mixed with glue, baked it on to set the glue, and then sprayed more of it on, without the glue.

Do that with scrambled eggs and you won't get them off either.
So what you're saying is that the non-stick is actually sticky - I'm going to take it back it and demand a refund!! (And the eggs become their problem...)
.
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  #746  
Old 12.06.2009, 13:58
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Re: Ask a Scientist

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Archemides principle says:
This principle states that a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid it displaces.Using this principle, it would have been possible to compare the density of the golden crown to that of solid gold by balancing the crown on a scale with a gold reference sample, then immersing the apparatus in water. If the crown was less dense than gold, it would displace more water due to its larger volume, and thus experience a greater buoyant force than the reference sample. This difference in buoyancy would cause the scale to tip accordingly.

So doesn't it mean that the coton has become lighter on air, to the amount of weight of same volume air?
But there you have it! It's not a gravitational force, so it doesn't change the weight of the body, just counters the usual effects of that weight (scales tipping, apples falling to the ground, etc.)

Suppose instead of immersing it in water, you put a powerful magnet under the scale. It would attract the iron and not the cotton. Would that mean that the iron was heavier, or that the scale was being interfered with?

Same thing with the Archimedes example - as soon as there are other forces at work besides gravity, the scale is no longer measuring just weight.
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  #747  
Old 12.06.2009, 14:12
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Re: Ask a Scientist

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Assuming a solid-ish lump of each, sitting on a table: iron, just slightly.

Iron is denser, so it's a smaller lump, so its centroid is closer to the earth's, so the gravitational forces between them are greater - in other words, it's heavier.

It's nowhere near a measurable amount though.
But a quick fag-packet calculation, taking the earth's radius as
6,371.009Km, and assuming that the cotton's centroid is 10cm
higher than the iron's, gives the difference in the two gs experienced
as
3.139220201534954486002108837001e-8
(or thereabouts).

If that's not measurable, then I'm very disappointed in the gravitationalists ,
cos the quantum dynamicists are up there (down there) at 10e-12 ,
clearly streets ahead (to use the SI unit of kudos).
.
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  #748  
Old 12.06.2009, 16:05
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Re: Ask a Scientist

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3.139220201534954486002108837001e-8
(or thereabouts).

If that's not measurable, then I'm very disappointed in the gravitationalists
I get the same result...

According to the German Wikipedia, modern gravimeters have a precision of up to 10^-9 of Earth's gravitation.

I've also found a small gravitation map for Switzerland:



People in the Eastern Alps have more mass than they think, gravitation is 0.002 m/s^2 less there
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  #749  
Old 12.06.2009, 18:29
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Re: Ask a Scientist

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People in the Eastern Alps have more mass than they think, gravitation is 0.002 m/s^2 less there
Their mass is constant wherever they go, it's less weight that they have.

Mass is the same irrespective of whether they are at sea level, up a mountain, or in the microgravity of earth orbit. Weight changes because it is the force exerted by gravity on a mass, and gravity changes according to the inverse square law (if I remember my school physics lessons correctly - damn, that was a long time ago...)

Didn't I read earlier in this thread about how much fun nitpicking was?

Last edited by Dave874y; 12.06.2009 at 18:30. Reason: less weight, not more you idiot Dave!
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  #750  
Old 12.06.2009, 23:14
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Re: Ask a Scientist

Well yes, I meant to say they experience less gravitational force per mass. The reason by the way is that the Alps fold towards the earth's center too, pushing back the more dense mantle of the earth.

Swissbob's question is still unanswered. I think that a compass needle's or bar magnet's total force on the underlay. One magnetic pole experiences a vertical "up" force (vector) and the other pole experiences the exact opposite.
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  #751  
Old 13.06.2009, 17:03
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Re: Ask a Scientist

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Putting salt on the roads lowers the freezing point of water. With the right amount of salt, the water stays liquid instead of freezing.

But it isn't entirely straightforward. Given the wrong temperature conditions, not enough salt can lower the freezing point just enough to make ice/snow melt during the day but freeze again at night, but now it's smooth sheet ice and very dangerous.
This quote brings me to a question raised in an Evangelical Meeting: How
come there are icebergs in the salty seas in the north and these icebergs are towed to the Mid-East and tapped as fresh water-supply to lands, yet
the icebergs do not get dissolved in the sea!
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  #752  
Old 13.06.2009, 18:06
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Re: Ask a Scientist

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This quote brings me to a question raised in an Evangelical Meeting: How
come there are icebergs in the salty seas in the north and these icebergs are towed to the Mid-East and tapped as fresh water-supply to lands, yet
the icebergs do not get dissolved in the sea!
'Cos it's pretty darn cold where icebergs are formed - far below the freezing point of seawater. As it turns out, frozen seawater divests itself of salt over time, so after a period of some years, it can be melted and drunk. You also have glaciers tipping out into the icecap, so you can get chunks of freshwater ice inbetween floes of frozen seawater.
See - reading Ice Station Zebra does pay off...
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  #753  
Old 14.06.2009, 16:36
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Re: Ask a Scientist

Brilliant thread, glad it got bumped yesterday and I got a chance to read through it!
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  #754  
Old 15.06.2009, 11:00
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Re: Ask a Scientist

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...
I've also found a small gravitation map for Switzerland:
...
Goodness, I haven't seen one of those for a long time! It's a
Bouguer Anomaly Map and the name is always good for a
schoolboy-humour snigger
.
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  #755  
Old 15.06.2009, 11:38
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Re: Ask a Scientist

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Here's what I learned from a recent episode of West Wing. It comes from a Eurocentric mindset that size is an indication of importance, and Up is better than Down. In many maps, Europe is larger than South America, and Greenland is larger than Africa.

According to the Peters Project, this is a more proportionally correct Map:
The area representation is not quite so simple, the trouble lies in trying to represent areas of a globe on a flat surface, there are actually many different ways of doing this, called map projections, but none will give a true representation of what countries actually look like. In the example above, the actual shapes of the countries are horribly distorted.

The earliest standard way of represent maps was the Mercator Projection, and it's still the most common used. Although invented by a Flemish guy, his reasons for using it we're not to make europe look bigger, but to aid accurate navigation, it's also a relatively easy one to work out. According to wikipedia, it isn't generally used in atlases any more.

Most countries maps are generated with the idea that they are the centre of the world (although north is still up, and south is still down), this isn't national pride, simply that it's the most accurate way to do it. Larger countries can also have multiple map projections.

To make things harder, the earth isn't a perfect a sphere, and we have different definitions of north.
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  #756  
Old 15.06.2009, 11:50
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Re: Ask a Scientist

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How come there are icebergs in the salty seas in the north and these icebergs are towed to the Mid-East and tapped as fresh water-supply to lands, yet the icebergs do not get dissolved in the sea!
Well, they do, but it takes longer than one might imagine. Most of the iceberg is under water, so the bulk of it remains protected from the warm surface water layer (but towing an iceberg would of course churn up the temperature layers).

The SOURCE of the icebergs is rainwater/snow, i.e. evaporated seawater, so icebergs are made of water without (much) salt.

Incidentally, I am not aware that this has ever been done in reality. To the best of my knowledge, it is a proposal only. Anyone know any more?

Edit: there is a patent which involves placing the entire iceberg in a huge sealed bag, so any melted fresh water is retained.

Last edited by resident; 15.06.2009 at 12:00.
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  #757  
Old 15.06.2009, 12:11
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Re: Ask a Scientist

I think the point of the above map is that the equator is centered. Which is a waste of blue ink because the majority of land is in the northern hemisphere.

weejeem, do you work in Geology or why do you know the name of this "Bouger Anomaly Map"? The map I linked by the way was measured from airplanes flying at 5500m altitude.
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  #758  
Old 15.06.2009, 13:18
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Re: Ask a Scientist

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but none will give a true representation of what countries actually look like. In the example above, the actual shapes of the countries are horribly distorted.
Right. The Peters Project is meant to represent land mass proportion while acknowledging that shape distortion is unavoidable.
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  #759  
Old 15.06.2009, 13:19
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Re: Ask a Scientist

To continue the theme of maps (well sort of). How big would Switzerland be if it was flattened out?
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  #760  
Old 15.06.2009, 13:30
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Re: Ask a Scientist

I have another q. on map and surveying.

When they say CH is ex. 41000 KM2, is this supposing country is flat or does it take all the terrain into account like all the mountains and valleys, which makes it much bigger?
If first choice, then CH will be bigger in reality for ex. compared to Denmark which is very flat and about the same area size. So this value is not very reliable.
If second choice, how you can accurately measure all the bumpy and complicated surfaces and ups and downs and turns?
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