[neilep]

quote:Originally posted by NakedScientist

Here's this week's QOTW

"WHAT MAKES YOUR JOINTS 'CRACK' FROM TIME TO TIME ?"

TNS

I heard once that it was something to do with a build up of gas that 'pops'....but knowing my history of answering these questions I'm bound to be wrong......hmmm...defeatist or what ?

'Men are the same as women...just inside out !'

[DrPhil]

joint surrounded by fluid
overextending joint = decrease in pressure
decrease in pressure = cavitation
cavitation = bubbles
bubbles burst = noise

[NakedScientist (OP)]

ANSWER TO LAST WEEK'S QOTW :

"WHAT MAKES YOUR JOINTS 'CRACK' FROM TIME TO TIME ?"

Dr. Phil has the correct answer. Joints are mobile articulations between bones. The ends of the bones are covered by a slippery layer of cartilage, rather like anatomical teflon, which is lubricated by a thin liquid called synovial fluid. The joint is enclosed by membranes and supporting tissues that retain and maintain the fluid, stabilise the joint, and also help to determine the directions in which it can move.

Because the fluid is held within an enclosed space, when the joint moves in certain directions it sometimes squashes the fluid on one side of the joint, and creates a partial vaccuum in the fluid on the other side of the joint.

Just as water boils at a lower temperature at the top of a mountain because the atmospheric pressure is lower at altitude, lowering the pressure in joint fluid causes small vapour bubbles to form (from the water in the synovial fluid). When these bubbles then subsequently collapse on themselves again they do so with a 'pop', which is the sound you hear.

This process is referred to as 'cavitation', and is responsible for the 'pitting' effect you see on boat propellers and hydrofoils. When the propeller cuts the water it creates zones of low pressure which yield vapour bubbles. The collapse of these bubbles against the propeller surface releases energy which damages the blades.

Fortunately for us, it happens too infrequently to cause harm to our joints !

TNS

[NakedScientist (OP)]

HERE'S THIS WEEK'S QOTW :

"WHY DOES ICE FLOAT, WHEN MOST SOLIDS ARE HEAVIER THAN LIQUIDS ?"

TNS

[neilep]

cos water is well weird and when it feezes it becomes less dense than liquid water... i know something strange happens at 4 degrees C, something do do with a crystal lattice and hydrogen ......and HEY !!!... I kind of got the above  (joints cracking etc)question partially right in my own way.....HMMMPTHH !!!


'Men are the same as women...just inside out !'

[gsmollin]

Yea, Neil just can't get no respect. He hits the buzzer first and credit goes to some guy with a "Dr" moniker. Maybe if he put his answer in the form of a question...

[neilep]

quote:Originally posted by gsmollin

Yea, Neil just can't get no respect. He hits the buzzer first and credit goes to some guy with a "Dr" moniker. Maybe if he put his answer in the form of a question...

Thanks for the support gsmollin !!...I feel so neglected sometimes []!!...but you've cheered me up...thanks.....

'Men are the same as women...just inside out !'

[tups]

Water in its liquid form forms networks and chains of water molecules that are connected together by hydorgen bonds, a kind of chemical bond in which a hydrogen atom is "shared" between two more electronegative atoms. Water always does a balance act : on the one hand thermal motion makes the networks smaller and expands the volume a single water molecule needs, ie regular thermal expansion. However, in an ideal network, every water molecule binds to two others with its hydrogen atoms, and accepts bonds from two more water molecules, and so forms a tetrahedric structure, which is very loosely packed, has, in other words, quite large holes in it. This ideal lattice is solid crystalline ice. It is thus less dense than when the networks break down into smaller and smaller chunks. The maximum density is reached at 4 degrees, where decreased networking starts to be offset by thermal expansion.
And that's the story of water. If it didn't form such a weird crystal structure, ice wouldn't float, oceans would regularly have frozen completely during the history of earth, and we wouldn't be alive today.
Lucky us ... :-)

tups

[Donnah]

quote:Originally posted by NakedScientist

ANSWER TO LAST WEEK'S QOTW :

"WHAT MAKES YOUR JOINTS 'CRACK' FROM TIME TO TIME ?"

Dr. Phil has the correct answer. Joints are mobile articulations between bones. The ends of the bones are covered by a slippery layer of cartilage, rather like anatomical teflon, which is lubricated by a thin liquid called synovial fluid. The joint is enclosed by membranes and supporting tissues that retain and maintain the fluid, stabilise the joint, and also help to determine the directions in which it can move.

Because the fluid is held within an enclosed space, when the joint moves in certain directions it sometimes squashes the fluid on one side of the joint, and creates a partial vaccuum in the fluid on the other side of the joint.

Just as water boils at a lower temperature at the top of a mountain because the atmospheric pressure is lower at altitude, lowering the pressure in joint fluid causes small vapour bubbles to form (from the water in the synovial fluid). When these bubbles then subsequently collapse on themselves again they do so with a 'pop', which is the sound you hear.

This process is referred to as 'cavitation', and is responsible for the 'pitting' effect you see on boat propellers and hydrofoils. When the propeller cuts the water it creates zones of low pressure which yield vapour bubbles. The collapse of these bubbles against the propeller surface releases energy which damages the blades.

Fortunately for us, it happens too infrequently to cause harm to our joints !

TNS

Does that mean that someone whose joints crack every time they bend them is more likely to have joint problems later in life?

"Remember, if you ever need a helping hand, you will find one at the end of each of your arms." - Audrey Hepburn

[chris]

I haven't heard any evidence that cracking joints are more prone to arthritis later in life, but I'll look into that.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[NakedScientist (OP)]

ANSWER TO "WHY DOES ICE FLOAT, WHEN MOST SOLIDS ARE HEAVIER THAN LIQUIDS ?"

Tups and Neil have this correct.

Water molecules resemble tiny boomerangs with an oxygen atom at the centre, and 2 hydrogens for arms. Because the oxygen likes electrons it pulls the electrons of the hydrogen atoms towards it, making the arms of the boomerang slightly positive, and the oxygen slightly negative.

This makes the molecules 'sticky' due to a process called hydrogen bonding. The slightly positive hydrogen from one water molecule is attracted to the slightly negative oxygen of another water molecule, and the 2 try to get closer together.

As you cool down water, the molecules lose energy, slow down, and can pack together more tightly. At 4 degrees C the molecules are at their most tightly packed.

But if you lower the temperature further it becomes more energetically favourable for the water molecules to arrange themselves into a more open tetrahedral lattice, in which each water molecule has a relationship with 4 others.

This configuration, rather like Swiss cheese, has lots of holes between the molecules and so the only way to pack in all the water molecules as freezing occurs is to make a crystal that takes up more volume than the starting liquid.

So why does it float ?

This is down to the Archimedes principle. When you place an object in water it displaces a volume of water equivalent to its weight. In other words water molecules push upwards on it with a force equal to the weight of the water that has pushed out of the way (displaced).

As ice is 'holey', as outlined above, it is much less dense than liquid water (about 9% less dense in fact) meaning that it doesn't have to sink very far before it has displaced enough water to support itself (because the deeper you go the higher the pressure and hence the higher the density).

As a result ice floats - thankfully - otherwise fish would be in real trouble, and the floors of our oceans would be covered with ice...

TNS

[NakedScientist (OP)]

Here's this week's QOTW

"HOW DO WE KNOW HOW FAR IT IS TO DISTANT GALAXIES - SUCH AS THE ANDROMEDA GALAXY WHICH IS 3 MILLION LIGHT YEARS AWAY ?"

[nilmot]

Just some chemistry fact about what 'NakedScientist' said above.

'The oxygen likes electron bit', the technical term is electronegative. Definition is the ability for a covalently bonded atom to draw a pair of electron toward itself. I think saying likes electron can be slightly misleading, some might mis-interpret it thinking atoms have preference to which electrons they like.

Tom

[neilep]

I won't answer this cos I'm proud to say I know the answer thanks to Dr Chris and Radio 5 Live :-)

'Men are the same as women...just inside out !'

[NakedScientist (OP)]

I'm sad that no one wants to have a go at this week's QOTW...[]

[Ultima]

Erm... Don't you take an observable object with known brightness within the Galaxy such as a supernova or I remember something about Cepheid Variables that have fixed intensity for their period which you can use, then using loads of rules of distance r and light intensity etc. you can work out the rough distance??? Or is this just rubbish? I'm guessing... my Astronomy module was a while ago []

wOw the world spins?

[nilmot]

Is it the Red Shift...?

My physic is really bad I have to say

Tom

[DrPhil]

I agree with Ultima. Back in the olden days (early 1900s) the luminosity of Cepheids was used as a yardstick, unfortunately they are not bright enough to be seen beyond our local galaxies. Now, the apparent brightness of the Type Ia supernova is used to determine distances to galaxies billions of light-years from earth. Last year astronomers discovered a pattern in the energy emitted by gamma-ray bursts which they believe could help them map the most distant parts of the cosmos.

[gsmollin]

quote:Originally posted by NakedScientist

I'm sad that no one wants to have a go at this week's QOTW...[]

I was inattentive, since the "question of the week" never comes out weekly, so I missed this week's (month's?) QOTW...

Historically, the Cepheid variables were used as brightness standards to show that the stars in the galaxy in Andromeda were much further away than thought before the time of Edwin Hubble. I believe the Cepheids were calibrated using parallax methods, although I should be checking my history before I make that claim. Hubble was then able to calibrate the red shift of the spectrum in the Cepheids, and showed the correlation between that and the distance. He then applied the red-shift-distance correlation to very distant galaxies, and determined they were billions of light years away. Of course, the red-shift-distance correlation factor became known as Hubble's constant, and has been refined since that time.

Later: According to the history, the Cepheid variables are named after the star Delta Cephei, which were studied by Henrietta Leavitt. D. Cephei is close enough to use a parallax measurement for its distance. With that, plus a knowledge of the period-absolute magnitude relationship in the Cepheid variables, Leavitt used the Cepheid variables to measure the distance to the Small Magellanic Cloud.

[simeonie]

if we had wings and a really strong chest could we fly?

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