For engineering/physics students/experts...
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keke. Let's see if anyone can explain the answer to the following question..hehe
Formula from our textbooks: Stress (N/m*m)= force (N) / area (m*m), where Stress is the force exerting on the area of contact.
So if one have a perfect sphere sitting on a perfect flat surface, the sphere touches the flat surface only at one single point, is equalivant to saying the the area of contact approaches zero. Because the one single point is so so so so so small.
hence stress = force / 0 or a very very small number (0.000000000000001). meaning stress becomes very very very big for a fix force.
Hence am I right to say as stress approaches infinity and if apply to real life everything will disintegrate? But in real life things are not disintegrating...so is the forumlar true??
Catch what I'm saying??
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If that thing is static, then the N will be finite.
Even though the area of the perfect sphere touching/contact with the surface is very small, it's still consider a positive value/interger.
Maximum Stress will then be the amount of Weight(Not mass) apply on the surface plus value of other factors such as atmospheric pressure, temperature, gravitational force, size of sphere, density of sphere and etc etc
*provided that the surface is able of withstand the stress apply to that particular point without changing it's physical shape.
And every real life or matter disintergrate under certain condition not limited to stress only.
Therefore formula is true.
Very good, know how to ask question. I muackx you
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Hmm... so if have a accurate machined grade A steel sphere (say total mass of 0.01kg) and placed it on a flat plastic (acrylic) surface what happens? Both surfaces are definately able to withstand the stress apply to that particular point without changing it's physical shape. SO the more accurate the machining, the more accurate the sphere, the smaller the area of contact the larger stress....hence shouldn't all accurate instruments "disintegrate"?Originally posted by Darkness_hacker99:If that thing is static, then the N will be finite.
Even though the area of the perfect sphere touching/contact with the surface is very small, it's still consider a positive value/interger.
Maximum Stress will then be the amount of Weight(Not mass) apply on the surface plus value of other factors such as atmospheric pressure, temperature, gravitational force, size of sphere, density of sphere and etc etc
*provided that the surface is able of withstand the stress apply to that particular point without changing it's physical shape.
And every real life or matter disintergrate under certain condition not limited to stress only.
Therefore formula is true.
Very good, know how to ask question. I muackx you -
The red part is correct.Originally posted by airgrinder:Hmm... so if have a accurate machined grade A steel sphere (say total mass of 0.01kg) and placed it on a flat plastic (acrylic) surface what happens? Both surfaces are definately able to withstand the stress apply to that particular point without changing it's physical shape. SO the more accurate the machining, the more accurate the sphere, the smaller the area of contact the larger stress....hence shouldn't all accurate instruments "disintegrate"?
For the blue part, the concept is right but put to application is a very different thing.
For blue part, you're talking about the "The ability to withstand stress"
Different ability to withstand stress can be explained by many factors such as thickness but at molecular level, it's they bonding which gives them the special property.
Every (solid)objects have different properity which makes them unique to withstand different types of stress.
If you reverse the dimension say 1 tonne of metal Sphere ball (diameter = 2m) is put on top of flat plastic acrylic of 1cm thickness by 2m x 2m, for sure to say that the plastic is unable to withstand the stress applied by the sphere ball. The plastic would definitely be bended and broke because it has past its point of flexibility and amount of stress created to withstand.
Of course there is a lot more factor to consider such as the distribution of force on plastic and etc.. -
I am surprised so much has been discussed without giving consideration to how strain actually helps to "cushion" the stress.
To illustrate this point, imagine a rectangular block of plasticine(for the sake of this example, let's assume its density is uniformly distributed). When stress is applied, what normally results would be a decrease in the height (ie.the dimension between the point where the external force is applied and the side of the plasticine in contact with the ground) of the plasticine, but the length and width (ie.it will become longer by the sides) would actually increase to "cushion" this external force.
Hence, that probably explains why your matter (plasticine in this case) will not collapse at a heap as Strain = Stress/Young's Modulus. As Darkness has already mentioned, the factors that determine the durability(ie.for how long it will conform to the principles as determined by your equation denoting stress) of an object have many possibilities - from atmospheric pressure to the density and Young's modulus of an object. As you can imagine from the above example given, should the stress be excessively greater than the permissible Young's modulus, what happens would result in the plasticine becoming totally flat(ie.height --> 0) with its length and width lengthened greatly...
Hope that clarifies the concept.
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With darkness here....guess I am not needed here anymore....
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Originally posted by Hamiltonian1125:With darkness here....guess I am not needed here anymore....
ok... I go 
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See, thats why I didn't majored in mechanics.
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no, cuz the areas surrounding the point of contact will apply a force to support the point of contact, if its just one thin line supporting the sphere then it probably will break.Originally posted by airgrinder:keke. Let's see if anyone can explain the answer to the following question..hehe
Formula from our textbooks: Stress (N/m*m)= force (N) / area (m*m), where Stress is the force exerting on the area of contact.
So if one have a perfect sphere sitting on a perfect flat surface, the sphere touches the flat surface only at one single point, is equalivant to saying the the area of contact approaches zero. Because the one single point is so so so so so small.
hence stress = force / 0 or a very very small number (0.000000000000001). meaning stress becomes very very very big for a fix force.
Hence am I right to say as stress approaches infinity and if apply to real life everything will disintegrate? But in real life things are not disintegrating...so is the forumlar true??
Catch what I'm saying?? -
I beg to differ. As per your earlier explanation, the formular holds true, but your explanation is not fully correct.Originally posted by Darkness_hacker99:The red part is correct.
For the blue part, the concept is right but put to application is a very different thing.
For blue part, you're talking about the "The ability to withstand stress"
Different ability to withstand stress can be explained by many factors such as thickness but at molecular level, it's they bonding which gives them the special property.
Every (solid)objects have different properity which makes them unique to withstand different types of stress.
If you reverse the dimension say 1 tonne of metal Sphere ball (diameter = 2m) is put on top of flat plastic acrylic of 1cm thickness by 2m x 2m, for sure to say that the plastic is unable to withstand the stress applied by the sphere ball. The plastic would definitely be bended and broke because it has past its point of flexibility and amount of stress created to withstand.
Of course there is a lot more factor to consider such as the distribution of force on plastic and etc..
Let's talk about simple application, and let me put in the numbers.
A light steel ball say 0.01kg resting on the LCD display of my nokia phone. I'm sure the LCD wun break.
But if the formula holds true, an I right to say that the more accurately the steel ball is machined to a perfect sphere, the less the area of contact (as ideally a ball touches a flat surface at only 1 dot single point.)
Say if the area of contact is say only 0.000 000 001 m*m
Stress = 0.01kg * 9.81 (gravity) / 0.000 000 001
=98,100,000 N/m*m
=98 MN/m*m
(For comparison Aluminium's young modules is 70 MN/m*m)
So in this case, why didn't the LCD screen crack when the ball is placed onto it? -
whether a material break or not oso depends on whether it is brittle or ductile...
Different materials got diff properties...
For more info borrow material science textbook.
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Read my earlier post for a better grasp of the problem. You cannot adequately understand this problem without first relating strain with stress and the "breaking point". For more details, refer to http://www.rwc.uc.edu/koehler/biophys/2f.html . Specifically, pay attention to the stress-strain correlation graph...Originally posted by airgrinder:I beg to differ. As per your earlier explanation, the formular holds true, but your explanation is not fully correct.
Let's talk about simple application, and let me put in the numbers.
A light steel ball say 0.01kg resting on the LCD display of my nokia phone. I'm sure the LCD wun break.
But if the formula holds true, an I right to say that the more accurately the steel ball is machined to a perfect sphere, the less the area of contact (as ideally a ball touches a flat surface at only 1 dot single point.)
Say if the area of contact is say only 0.000 000 001 m*m
Stress = 0.01kg * 9.81 (gravity) / 0.000 000 001
=98,100,000 N/m*m
=98 MN/m*m
(For comparison Aluminium's young modules is 70 MN/m*m)
So in this case, why didn't the LCD screen crack when the ball is placed onto it? -
Hi walesa and the.owl.
Both of you got it correctly.
I knew the answer. I just here to "explode" engineering /physics people minds about learning but not understanding and not applying.
As what I had posted above, it I place a sphere on a flat surface, strictly following stress=force/area, stress would be a infinate big number, and everything will disintegrate.
However in real life this is not true. There is a young modulus E, the relation between stress and strain.
The first formular do not hold true solely by itself. Because in the first place deformation WILL occur at the very instant when I place the sphere on the LCD screen, and hence the area of contact can NEVER be one single dot.
There will be a valid area of contact hence stress is a vaid number, and things dun disintegrate.
Can go test your engineering friends about this. My experience, most will be shock!! "huh, ya hor, like that why didn;t thee things disintegrate huh??"
keke -
Well said.Originally posted by the.owl:no, cuz the areas surrounding the point of contact will apply a force to support the point of contact, if its just one thin line supporting the sphere then it probably will break. -
Yup yup. Totally agreeded.Originally posted by walesa:Read my earlier post for a better grasp of the problem. You cannot adequately understand this problem without first relating strain with stress and the "breaking point". For more details, refer to http://www.rwc.uc.edu/koehler/biophys/2f.html . Specifically, pay attention to the stress-strain correlation graph...
For more information, one can also look for "Contact Mechanics"