Fluid Mechanics and perpendicular force

In summary: The lateral surfaces of the body will experience a perpendicular force due to the fact that the water is pushing down on the surface.
  • #1
Badfish97
15
0
I was just wondering why a body immersed in a fluid at rest experiences a perpendicular force on its surface? Why can't there be a component of force in parallel direction to the surface ? I read somewhere that this would cause the fluid to flow parallel to the surface but i don't quite understand...can someone please explain.
PS I'm new.
 
Physics news on Phys.org
  • #2
Badfish97 said:
I was just wondering why a body immersed in a fluid at rest experiences a perpendicular force on its surface? Why can't there be a component of force in parallel direction to the surface ? I read somewhere that this would cause the fluid to flow parallel to the surface but i don't quite understand...can someone please explain.
PS I'm new.

How does the force know which way to point in the parallel direction?
 
  • #3
Two reasons off the top of my head.
1. If all the forces parallel to the surface were in the same direction (difficult if it is a 3d body) then there would be a net torque on the body and it would rotate. By observation there is no rotation so no nets torque.
2. If the forces parallel to the surface were not in a particular direction relative to the surface what would cause them to be in different directions? The only possibility with regard to the symmetry of the system is if the forces parallel to the surface were zero.
 
  • #4
bhillyard said:
Two reasons off the top of my head.
1. If all the forces parallel to the surface were in the same direction (difficult if it is a 3d body) then there would be a net torque on the body and it would rotate. By observation there is no rotation so no nets torque.
2. If the forces parallel to the surface were not in a particular direction relative to the surface what would cause them to be in different directions? The only possibility with regard to the symmetry of the system is if the forces parallel to the surface were zero.

Nicely analyzed.

Chet
 
  • #5
Badfish97 said:
I was just wondering why a body immersed in a fluid at rest experiences a perpendicular force on its surface? Why can't there be a component of force in parallel direction to the surface ? I read somewhere that this would cause the fluid to flow parallel to the surface but i don't quite understand...can someone please explain.
PS I'm new.

When there is a molecule(atom) of the fluid that touches the body, the forces in all directions are in equilibrium (it is surrounded with the same molecules(atoms)), except the direction to the surface of the body.

So the only unbalanced force will be in direction perpendicular to the surface of the body (forces between fluid and body are not the same as this between fluid molecules)

bouy.gif


http://hyperphysics.phy-astr.gsu.edu/hbase/pbuoy.html
 
Last edited by a moderator:
  • #6
Makes sense! But why does the force experienced on the surface have to be perpendicular? I can understand if the body immersed is a cube or a cuboid...but what about a body with an inclined edge..like a prism?
 
Last edited by a moderator:
  • #7
Badfish97 said:
Makes sense! But why does the force experienced on the surface have to be perpendicular? I can understand if the body immersed is a cube or a cuboid...but what about a body with an inclined edge..like a prism?

Because, when fluid molecules are pushed from all sides from their neighbors. The only place they can go, is the volume of the body.
So they press it from all sides.
As I said, in all other directions, there are fluid molecules.
The only direction that molecule get no force from other fluid molecules, is that which points to the body surface.
 
  • Like
Likes 1 person
  • #8
Malverin said:
Because, when fluid molecules are pushed from all sides from their neighbors. The only place they can go, is the volume of the body.
So they press it from all sides.
As I said, in all other directions, there are fluid molecules.
The only direction that molecule get no force from other fluid molecules, is that which points to the body surface.
Yes...I understand..Thanks :)
 
  • #9
Badfish97 said:
I was just wondering why a body immersed in a fluid at rest experiences a perpendicular force on its surface? Why can't there be a component of force in parallel direction to the surface ? I read somewhere that this would cause the fluid to flow parallel to the surface but i don't quite understand...can someone please explain.
PS I'm new.
Gravity it act's in a downwards direction.A fluid surrounding an object which is immersed has gravity acting upon it in that same direction.Gravity does not attract in a parallel direction.
 
  • #10
Buckleymanor said:
Gravity it act's in a downwards direction.A fluid surrounding an object which is immersed has gravity acting upon it in that same direction.Gravity does not attract in a parallel direction.
What about the horizontal surfaces of the body? Gravity does explain the force on the vertical surfaces (considering a cube), on the top face. The bottom face force can be explained by buoyancy...but what about the force experienced by the lateral faces (3-D)? I am referring to force perpendicular to the face of course.
 
  • #11
Badfish97 said:
Makes sense! But why does the force experienced on the surface have to be perpendicular? I can understand if the body immersed is a cube or a cuboid...but what about a body with an inclined edge..like a prism?
Dear Badfish97. You still haven't answered my question for my earlier post. When you can answer that question, you will know the answer to your question. So, here it is again:

In a static fluid, if there were a force on a surface that is not perpendicular to the surface, the force could be resolved into components perpendicular and tangent to the surface. The tangent component would have a choice of directions within the tangent plane that span 180 degrees. So, if the fluid is static, how would you think that the force would know in what direction it should be pointing within this plane?

As far as your question about fluid forces acting on vertical surfaces is concerned, at a given location in a static fluid, the pressure acts equally in all directions, including horizontally on vertical surfaces. This means that on a surface oriented in any arbitrary direction, the pressure stress always acts normal to the surface. This is an experimentally observed fact that was discovered by Pascal.

Chet
 
  • #12
Chestermiller said:
Dear Badfish97. You still haven't answered my question for my earlier post. When you can answer that question, you will know the answer to your question. So, here it is again:

In a static fluid, if there were a force on a surface that is not perpendicular to the surface, the force could be resolved into components perpendicular and tangent to the surface. The tangent component would have a choice of directions within the tangent plane that span 180 degrees. So, if the fluid is static, how would you think that the force would know in what direction it should be pointing within this plane?

As far as your question about fluid forces acting on vertical surfaces is concerned, at a given location in a static fluid, the pressure acts equally in all directions, including horizontally on vertical surfaces. This means that on a surface oriented in any arbitrary direction, the pressure stress always acts normal to the surface. This is an experimentally observed fact that was discovered by Pascal.


Chet

Well if I understood your question correctly, you're asking me the direction of the tangential component of force when it is not perpendicular to the surface. I think that would depend on the direction of the applied force. for eg: (see attachment).

In reference to your explanation, so the surface of the body experiences a force in all direction, but we only consider the normal force acting on it...given this explanation, I'm assuming all the other forces get nullified, so the net force is acting perpendicular to the surface. Am I correct?

PS - Sorry for not replying earlier. I didn't understand your question then.
 

Attachments

  • Untitled.png
    Untitled.png
    4.1 KB · Views: 890
  • #13
Badfish97 said:
What about the horizontal surfaces of the body? Gravity does explain the force on the vertical surfaces (considering a cube), on the top face. The bottom face force can be explained by buoyancy...but what about the force experienced by the lateral faces (3-D)? I am referring to force perpendicular to the face of course.
The gravitational force on the verticle surfaces is either unbalanced in one direction or another (top or bottom)depeding on the density of the immersed body, or in equilibrium, if it is static within the fluid.
The force acting on the lateral faces is allways equal no matter what the density or the position of the object.
 
  • Like
Likes 1 person
  • #14
Badfish97 said:
Well if I understood your question correctly, you're asking me the direction of the tangential component of force when it is not perpendicular to the surface. I think that would depend on the direction of the applied force. for eg: (see attachment).

Unlike a solid, a fluid can't support a tangential component of force at a surface under static conditions. It can if the fluid is shearing (deforming) at the surface (as a result of viscous stresses), but viscous stresses are not present under static conditions (i.e., the fluid is not deforming as time progresses).

In reference to your explanation, so the surface of the body experiences a force in all direction, but we only consider the normal force acting on it...given this explanation, I'm assuming all the other forces get nullified, so the net force is acting perpendicular to the surface. Am I correct?
.

No. I should have been more precise in what I said. When a fluid mechanics guy (I'm a fluid mechanics guy with lots of experience) says that the pressure at a point in the fluid acts equally in all directions, what he really means is that, for any arbitrarily oriented surface, the pressure only acts normal to the surface and is the same value irrespective of the orientation of the surface.

In an earlier post, Malverin provided the correct molecular explanation of why, under static conditions, the pressure force in the fluid (pressure stress) only acts perpendicular to the surface.

Chet
 
  • Like
Likes 1 person
  • #15
Chestermiller said:
In an earlier post, Malverin provided the correct molecular explanation of why, under static conditions, the pressure force in the fluid (pressure stress) only acts perpendicular to the surface.

Chet
What causes the pressure force in the fluid to act perpendicular to the surface.Is it because all the molecules are stacked one on top of another and gravity causes a pressure difference which is greater at the bottom of the container of fluid than the top.
So the pressuire stress points upwards.Or is it some other mechanism which I don't understand.
 
  • #16
Buckleymanor said:
What causes the pressure force in the fluid to act perpendicular to the surface.Is it because all the molecules are stacked one on top of another and gravity causes a pressure difference which is greater at the bottom of the container of fluid than the top.
So the pressuire stress points upwards.Or is it some other mechanism which I don't understand.
It's a combination of two things. Leaving out the molecular explanation, if you analyze the system at the continuum level, then, as you said, the pressure on both sides of a flat horizontal surface must be equal to the weight of the fluid above (plus the atmospheric pressure at the top). In addition to this, there is Pascal's experimental observation that, no matter which direction you orient a flat surface at depth, the pressure force per unit area on it will be normal to the surface and have the same value as when the surface is oriented horizontally. If you used the subsurface pressure to compress a little sphere of fluid, it would compress equally in all directions.

Chet
 
  • Like
Likes 1 person
  • #17
Chestermiller said:
Unlike a solid, a fluid can't support a tangential component of force at a surface under static conditions. It can if the fluid is shearing (deforming) at the surface (as a result of viscous stresses), but viscous stresses are not present under static conditions (i.e., the fluid is not deforming as time progresses).



No. I should have been more precise in what I said. When a fluid mechanics guy (I'm a fluid mechanics guy with lots of experience) says that the pressure at a point in the fluid acts equally in all directions, what he really means is that, for any arbitrarily oriented surface, the pressure only acts normal to the surface and is the same value irrespective of the orientation of the surface.

In an earlier post, Malverin provided the correct molecular explanation of why, under static conditions, the pressure force in the fluid (pressure stress) only acts perpendicular to the surface.

Chet

Given my understanding of vector resolution, the vertical component (Fsinx) of the force should be perpendicular to the horizontal component (Fcosx).I know the slightly delves into a different discussion all together, but plainly, how does resolution of a vector happen then for a static fluid?
i.e. What is the direction of the tangential component of the force?

So, correct me if my understanding is wrong. The reason the surfaces of a body immersed in a fluid
experience a perpendicular force is because the fluid at each point exerts equal pressure in all directions and the only direction where it does not encounter another fluid molecule is towards the surface of the body. Since it is surrounded by other fluid molecules in all directions except the surface and the shortest distance between the molecule in question and the surface is in the perpendicular direction, and since it is the only direction where it will encounter an unbalanced force, it exerts pressure perpendicular to the surface...?
 
  • #18
Badfish97 said:
Given my understanding of vector resolution, the vertical component (Fsinx) of the force should be perpendicular to the horizontal component (Fcosx).I know the slightly delves into a different discussion all together, but plainly, how does resolution of a vector happen then for a static fluid?
i.e. What is the direction of the tangential component of the force?

So, correct me if my understanding is wrong. The reason the surfaces of a body immersed in a fluid
experience a perpendicular force is because the fluid at each point exerts equal pressure in all directions and the only direction where it does not encounter another fluid molecule is towards the surface of the body. Since it is surrounded by other fluid molecules in all directions except the surface and the shortest distance between the molecule in question and the surface is in the perpendicular direction, and since it is the only direction where it will encounter an unbalanced force, it exerts pressure perpendicular to the surface...?
The momenta of the individual molecules striking the surface are in all different directions. However, the tangential components of the momenta all cancel each other out (statistically) if the fluid is static. But the normal components do not cancel each other out. This gives rise to a pressure force per unit area on the surface acting normal to the surface only.
 
  • Like
Likes 1 person

1. What is fluid mechanics?

Fluid mechanics is a branch of physics that studies the behavior of fluids, which include liquids and gases, in motion.

2. How is fluid mechanics related to perpendicular force?

Fluid mechanics is concerned with the forces acting on fluids, including perpendicular forces. Perpendicular force is a type of force that acts at a right angle to the surface of an object, and it is an important factor in determining the motion and behavior of fluids.

3. What is the difference between parallel and perpendicular forces in fluid mechanics?

In fluid mechanics, parallel forces act in the same direction as the flow of the fluid, while perpendicular forces act at a right angle to the flow. These forces can have different effects on the fluid, such as changing its direction or causing it to flow faster or slower.

4. How does the shape of an object affect the perpendicular force in fluid mechanics?

The shape of an object can greatly influence the perpendicular force in fluid mechanics. For example, a streamlined shape can reduce the perpendicular force and decrease drag, while a blunt shape can increase the perpendicular force and create more resistance.

5. What are some real-world applications of fluid mechanics and perpendicular force?

Fluid mechanics and perpendicular force are used in a wide range of applications, such as designing efficient aerodynamic shapes for airplanes, optimizing the flow of water in rivers and pipes, and developing hydraulic systems for heavy machinery. They are also important in understanding weather patterns and ocean currents.

Similar threads

Replies
16
Views
938
Replies
7
Views
780
Replies
5
Views
2K
Replies
3
Views
1K
Replies
73
Views
14K
Replies
13
Views
3K
Replies
17
Views
612
Replies
6
Views
1K
Replies
1
Views
2K
Replies
15
Views
6K
Back
Top