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My understanding of surface tension is that it acts perpendicular to an imaginary line on the surface of the liquid. i.e for a paper clip it will act horizontally, and in the same plane as the surface of water.

Gravity, however will act downwards, and the two forces in theory shouldn't balance each other right? Then how do paper clips float on water because of surface tension?

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For a paper clip floating on water, the surface of the water is not flat, but curves downwards so that surface tension acts upwards. This is essentially the effect which causes capillary action.

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It is similar to a membrane with a weight in the center. The weight will force a displacement down. The elastic membrane must increase its area to the new configuration.

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    $\begingroup$ The difference is that the surface tension force per unit length does not change by distortion of the surface. $\endgroup$
    – Chet Miller
    Commented May 29, 2020 at 22:32
  • $\begingroup$ @Chet Miller yes, you are right. I deleted the last paragraph. $\endgroup$
    – Claudio Saspinski
    Commented May 30, 2020 at 0:22

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