Cold-co Posted December 15, 2009 Report Posted December 15, 2009 Lifting a circular wire of length (l) out of a liquid requires a pull that is greater than the force needed to lift the wire’s weight, which as you know is a product of gravity’s vertical acceleration. In response to this phenomena, physicists introduce a surface tension constant (S) for each liquid; P = 2Sl, and credit the pull on both sides of the wire with resisting movement. In a schematic of forces diagram, this problem is similar to the rotational shift of mass that causes earth’s flattening—flattening that geodesists use to determine her moment of inertia. However, geodesists only consider the force on one side of the bulge in their equation. Shouldn’t the gravitational pull on each side of the bulge be considered? BrianG 1 Quote
lawcat Posted December 24, 2009 Report Posted December 24, 2009 That's not true. In the surface tension formula also, only one force is considered: the lifting force. The "2" comes from cosideration of two surfaces. For example, if you have a wire shaped in a U, and you put a cross bar, a wire, accross the the pins of the U, so that it looks like a D; and if you start stretching that cross bar I of the D with a force F; then you are pulling two surfaces, one on each side of the D. Thus, 2 surface tensions (S) exist, totaling, equaling, F/L of the crossbar. Thus the formula is 2S = F/L or S = F/2L.In a round ring model, which is to be pulled from the surface, similarly only one force is considered: the pulling force. But the surface of the water is on both sides, up to the air and down to the water. Thus, two surface tensions (2S), equal the F divided by 2r*pi; 2S = F/ 2*r*pi; or S = F / 4*r*pi. Quote
Cold-co Posted December 24, 2009 Author Report Posted December 24, 2009 Lawcat:That isn't how my physics text states the problem. Quote
lawcat Posted December 24, 2009 Report Posted December 24, 2009 Yes it is. You are not reading it right. You stated above that your physics books says: P = 2SL.P is the pulling force. S is not a constant, but surface tension, which is a linear force distribution. L is the length. I said above: S = F/2L, which is the same as your F = 2SL. Only one force, the pulling force. Quote
Cold-co Posted December 26, 2009 Author Report Posted December 26, 2009 Lawcat:I think we are talking about the same thing, but using different symbols. In a schematic forces diagram there are three opposing forces that P must overcome; m1g + m2g + mwg; m1 being the weight of the fluid on the right side of the wire, m2g being the weight of the fluid on the left side of the wire and mg being the weight of the wire. Now, we know fluids do not chemically bond, so the force holding the fluid together and preventing its separation must be a gravitational attraction. Two cubic centimeters of a fluid attract each other with a gravitational force that is equal to the gravitational constant times the square of their densities, which causes tension in the fluid. Surface tension is an experimentally demonstrated force per unit length, defined as the ratio of the surface force to the length along which the force acts. What I question is, "Why do geodesist not recognize these forces in their flattening equation?" Quote
lawcat Posted December 26, 2009 Report Posted December 26, 2009 Your question goes to this issue: Whether the Earth should be modeled as a liquid based on the fact that it flattens and if it should, then what is the surface tension of the Earth. That is a question that a Ph.D. physict should answer, or someone with more knowledge than me. Cold-co said: . In a schematic forces diagram there are three opposing forces that P must overcome; m1g + m2g + mwg; m1 being the weight of the fluid on the right side of the wire, m2g being the weight of the fluid on the left side of the wire and mg being the weight of the wire. No. Surface tension is linear distribution of "cohesive" forces on the surface of a liquid, and it has nothing to do with the weight.We use the pulling force to experimentally, empirically, determine the surface tensions. Then, based on that data, we can say whether certain weight or force acting on the liquid's surface will break the surface or whether the surface will be in tension. Cold-co said: Now, we know fluids do not chemically bond, so the force holding the fluid together and preventing its separation must be a gravitational attraction. No. First, The force holding the "fluid's" surface together is not gravitational force, but a molecular cohesive force. Second, Fluids are divided into gases and liquids. Gases are free and not bound. Liquids are bound. For gases, surface tension is infinite; it is not applicable. Conversley, for solids, surface tension is zero; it does not exist, it is not applicable. Surface tension is only applicable to liquids. And it does not come from gravitation but from molecular cohesive forces. Cold-co said: .Two cubic centimeters of a fluid attract each other with a gravitational force that is equal to the gravitational constant times the square of their densities, which causes tension in the fluid. Now you are talking about two liquids as gravitational objects in free space. If there are two liquids objects, each with gravitational field, acting on each other in free space; then, the two will attract each other depending on the strength of the gravitational field. If the gravitational force is not strong enough to merge the two liquids, but is stronger than the forces of surface tension, the liquid's surface will shear until the equilibrium is obtained. Cold-co said: .What I question is, "Why do geodesist not recognize these forces in their flattening equation?" They do not because surface tension does not affect the flattening. Surface tension only exists on the surface of a liquid. Earth's crust is a solid and is modeled as a solid, and solids do not exhibit surface tension. The water merely sits on the surface of the Earth. It is affected by the Moon's gravitation. However, the Earth's surface water is in equilibrium; meaning, it does not shear away due to Moon's gravitational force. Quote
Cold-co Posted December 28, 2009 Author Report Posted December 28, 2009 Lawcat: Since I'm unable to understand how a lateral force becomes a vertical restraint, I suggest we discontinue this thread. Thank you for your time. Quote
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