Go Back   Physics Help Forum > High School and Pre-University Physics Help > Thermodynamics and Fluid Mechanics

Thermodynamics and Fluid Mechanics Thermodynamics and Fluid Mechanics Physics Help Forum

Reply
 
LinkBack Thread Tools Display Modes
Old May 11th 2015, 02:22 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?
SurajBahuguna is offline   Reply With Quote
Old May 11th 2015, 06:03 AM   #2
Senior Member
 
Join Date: Apr 2015
Location: Somerset, England
Posts: 993
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
studiot is offline   Reply With Quote
Old May 11th 2015, 06:45 AM   #3
Physics Team
 
ChipB's Avatar
 
Join Date: Jun 2010
Location: Morristown, NJ USA
Posts: 2,310
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.
ChipB is offline   Reply With Quote
Old May 11th 2015, 06:59 AM   #4
Senior Member
 
Join Date: Apr 2015
Location: Somerset, England
Posts: 993
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.
studiot is offline   Reply With Quote
Old May 11th 2015, 09:20 AM   #5
Physics Team
 
ChipB's Avatar
 
Join Date: Jun 2010
Location: Morristown, NJ USA
Posts: 2,310
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.
ChipB is offline   Reply With Quote
Old May 11th 2015, 09:36 AM   #6
Senior Member
 
Join Date: Apr 2015
Location: Somerset, England
Posts: 993
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.
studiot is offline   Reply With Quote
Reply

  Physics Help Forum > High School and Pre-University Physics Help > Thermodynamics and Fluid Mechanics

Tags
acting, fluid, fluid dynamics, force, net, perpendicular, surface



Search tags for this page
Click on a term to search for related topics.
Thread Tools
Display Modes


Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
How to guide lights from all direction, straight and perpendicular to a surface? JJoll Advanced Optics 4 Nov 14th 2013 04:12 AM
Net force acting on a liquid dielectric omirs Advanced Electricity and Magnetism 0 Nov 10th 2012 09:18 AM
Force acting on charged parabola kiffren Advanced Electricity and Magnetism 0 Jun 4th 2012 08:19 PM
Coriolois Force Acting on Carousel quantum_enhanced Advanced Mechanics 0 Sep 20th 2011 04:23 PM
Resultant Force Acting On Springs : Confused cyt91 Equilibrium and Elasticity 4 Sep 17th 2010 06:48 PM


Facebook Twitter Google+ RSS Feed