Physics Help Forum Why surface of fluid is perpendicular to net force acting on it?
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 May 11th 2015, 03:22 AM #1 Junior Member   Join Date: May 2015 Posts: 1 Why surface of fluid is perpendicular to net force acting on it? I have been told by my teacher that the surface of a fluid is always perpendicular to the net force acting on it. The reason being a fluid can not withstand tangential stress and if a shear stress is applied to it, it will slip until the surface becomes perendicular to the net force. So my question is why the surface of water is horizontal in a vessl at rest even when the net force acting on it is zero? Also why does it become diagonal(and not vertical) when it is given some horizontal accleration?
 May 11th 2015, 07:03 AM #2 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 995 This sounds like a simplification of what is actually a complicated subject. The difference between pressure and force. It is necessary to understand this difference to understand any more detailed answer. It would be very helpful if you would indicate what level you are studying at, GCSE or A level? Some differences. Force is a vector Pressure is a scalar Force always has a moment about any point or line. Pressure may or may not exert a moment on something, but this is complicated Force has a defined line of action and direction Fluid pressure at a point is the same in all directions
 May 11th 2015, 07:45 AM #3 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,324 I think the term "net force" as used by your teacher is incorrect. Think about a molecule of water that is at the surface of stationary body of water. The forces acting on it are gravity pulling down plus pressure from the sides plus pressure from below. All those forces cancel, so the net force on the molecule is zero, and it is stationary (from F=ma). So to your point, the statement that the surface is perpendicular to "net force" is incorrect, as net force = 0. I think instead your teacher should have said: the surface of the fluid is always perpendicular to the net external force acting on the molecules at the surface. For the case of the molecules on the surface of a stationary body of water the net external force is directed downward (gravity), so the surface is horizontal. If the body of water is accelerating horizontally, then the forces acting on the molecules include gravity downward plus horizontal force of magnitude F=ma, where m = mass of the surface molecules and a= horizontal acceleration. The vector sum of gravity plus this force is at some angle to the vertical, and hence the surface of the fluid is at some angle to horizontal.
 May 11th 2015, 07:59 AM #4 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 995 I thought about the issue of net force and decided to duck it until more information was forthcoming. In particular I wondered if the OP misheard teacher. So the statement was actually that the net force is perpendicular to the surface, rather than the other way round, which makes more sense You can't have a net force without something for it to act on so that something must come first and that something must have a surface.
 May 11th 2015, 10:20 AM #5 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,324 studiot - I'm not sure I agree. An external force is first applied to the fluid, and the result is a surface. Consider for example a fluid with no external force acting on it at all - say a container of liquid in zero gravity. The fluid in that container doesn't form a surface, except at the boundary where the fluid molecules may bump into the wall of the container. This is perhaps a little simplistic, because in reality surface tension acts to keep a blob of liquid together in zero g, but if you could imagine a fluid with no surface tension in zero g it would simply be a cloud of molecules. So without an external force there is no surface.
 May 11th 2015, 10:36 AM #6 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 995 I think it's pointless arguing over the definition since the expression 'net force' could mean two different things and I am not sure which is meant. Equally the word 'surface' could mean different things. That is why I am trying to establish the level for a meaningful discussion with the OP.

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