# Surface Tension

## Main Question or Discussion Point

Hi PF!

Simple question here, but if we have some fluid in, say a cylinder container, with a specified surface tension, to get force would we multiply the surface tension by the height of the cylinder (if it's standing up on a circular side)? I know we multiply it by some length, but I don't understand it enough to know which one.

Josh

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Doc Al
Mentor
What force are you trying to find?

You know, I'm not really sure. I guess what I'm really wondering is what direction that length is, since surface tension is force per unit length.

Bystander
Homework Helper
Gold Member
Consider the concept of "surface energy." Think about that word, "surface."

Yea, my though is per unit height...is this wrong? Like, orthogonal to whatever surface we are talking about?

Doc Al
Mentor
Yea, my though is per unit height...is this wrong?
If you're trying to find the force exerted by the surface tension, height has nothing to do with it. You need to ask: Force on what?

I see what you're saying. To clarify my question entirely, I am reading a paper and there is a non-dimesnional velocity term. Now I could simply take the paper's word and use their proposed velocity, but I want to work it out and see for myself that they have correctly non-dimensionalized the velocity component. They introduce the surface tension for non-dimensional purposes. I am trying to find the direction of that length that is being used. Is this length in surface tension not always oriented in the same direction with respect to the fluid surface?

Doc Al
Mentor
I am trying to find the direction of that length that is being used. Is this length in surface tension not always oriented in the same direction with respect to the fluid surface?
When finding the force exerted by the surface tension on some line, that line is on the surface or its circumference. Perhaps if you posted more details someone will recognize exactly how the surface tension is being used in your application.