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When we put a little pin on the surface of water, it floats; is this because of surface tension or buoyancy? Can somebody also draw a force diagram for me to explain how surface tension of water supports an object. And anybody has any advise for me that I can do any simple experiment to demonstrate water-surface tension?

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  • $\begingroup$ For the simple experiment: you could float a pin in a bowl of water and then add some dish washing soap to make it sink. The reason that it sinks is that the surfactant molecules of the soap will lower the air-water surface tension $\endgroup$ – Michiel Jun 2 '14 at 5:26
  • $\begingroup$ I thought about this question before. Isn't it possible that the detergent( about 0.8g/cm3), which is less dense than water, is dissolved in water and cause the mixture's density less than water itself. So the pin sinks because of its greater density? $\endgroup$ – user40003 Jun 3 '14 at 13:12
  • $\begingroup$ and I still don't understand how water screen works. $\endgroup$ – user40003 Jun 3 '14 at 13:34
  • $\begingroup$ the water screen trick is not only about surface tension right? Air pressure also exerts force on the water. $\endgroup$ – user40003 Jun 3 '14 at 13:46
  • $\begingroup$ The effect of the lower density of the detergent is negligible because you will only need a tiny amount $<1\%$ of detergent for the pin to sink. Check the wiki on surfactants if you want to known more $\endgroup$ – Michiel Jun 4 '14 at 5:12
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The pin floats on the surface of water because of water's surface tension. When a pin is delicately placed on the surface of still water, it creates a small depression on the water's surface. If the pin is of unit length, then through out its length, the water's surface experiences a force T. If $\alpha$ is the angle of depression, then there is a net upward force $2T\sin\alpha$ that balances the pin's weight. You may refer to the figure for the meaning of symbols.

Force of a stretched membrane on a pin

The blue circle is the transverse section of the pin. That is, the pin is placed perpendicular to the screen.

The force $2T\sin\alpha$ is analogous to the normal reaction of a solid surface.

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  • $\begingroup$ Of course, assuming that the pin has a density higher then water. If it would have a density that is lower than that of water then the surface tension might still act, but the pin will not sink without it. $\endgroup$ – Michiel Jun 2 '14 at 5:24
  • $\begingroup$ what's the role of buoyancy here? $\endgroup$ – user40003 Jun 3 '14 at 13:15
  • $\begingroup$ Buoyancy plays a role when an object is fully of partially submerged in the fluid. In this case, the surface of fluid "bears the weight" of the object. Therefore, buoyancy does not come in the picture. $\endgroup$ – Amey Joshi Jun 3 '14 at 15:24
  • $\begingroup$ @AmeyJoshi , it is also true that the pin is displacing fluid due to the curvature or the surface. For objects like a small hydrophobic dense sphere, may that be a relevant contribution to the total force? $\endgroup$ – Andrestand Jan 22 '15 at 12:59
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Surface tension and the buoyancy force are both working together to keep the object to keep floating. Surface tension keeps light object from falling into the water; buoyancy pulls up the object.

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  • $\begingroup$ Please note that punctuation isn't supposed to be optional, it actually helps improve readability. $\endgroup$ – Kyle Kanos Oct 26 '15 at 10:15
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Surface tension provided by the surface molecules of water and its buoyancy together are responsible for the floating of the pin...they are generally related in such situations. Moreover Surface tensions is a combined factor of buoyancy in certain terms.

Hope that helps you in brief.

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When someone floats on water, it’s all done with the help of surface tension and buoyancy. When someone relaxes gently on water, it creates a small depression on the water's surface. Surface tension and buoyancy both work together to keep the person or object a float. Surface tension keeps the person or object from falling into the water; but buoyancy pulls up the person or object. Surface tension is made by the molecules in the water.

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protected by AccidentalFourierTransform Aug 30 '18 at 15:36

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