Why surface tension acts tangentially if there is a net inward force? [closed]








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I understand that surface tension is caused by an imbalance of forces for molecules on the surface in comparison to those that lie within. As a result, this creates a net inward force that seeks to reduce surface area. However, if this force acts inwards, why does surface tension act tangentially? I've seen explanations that state bonds on the surface become stronger when the surface area is minimised. If this is true then why is this the case? It doesn't seem clear to me why a net downward force causes the surface to act as an elastic membrane.




Each molecule at the surface of a drop of water is pulled towards the others around it (by cohesion, i.e. in this case hydrogen bonds), those at the surface, and those below. There is nothing above it, so the net force is pointing towards the center of the drop.


In the orthogonal surface area, the forces cancel out, except if the surface was previously disturbed/deformed, i.e. the area increased from the planar/spherical ideal.


When you increase the surface area by deforming the drop, that means you pull out water molecules from the body, against that net downwards force.


Ergo, surface tension and that perpendicular force are essentially the same thing.


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