Why drag force exerted qhen immerg into the liquid?
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As you have observed correctly, there is nothing at rest in nature. What we see at rest is only a statistical average, but the internal constituents- the molecules are never at rest. In fact the smaller you go the faster particles go.. so that molecules are made of atoms and these contain particles that move near the speed of light!
Pressure (normal stress) is force per unit area- this is the definition. Since force and area are vectors, the definition chooses the component that is normal to the surface. The force comes from the change in momentum of the agitated fluid particles as they hit the wall (in reality they never touch.. just deflected back by static forces). They come with a certain momentum normal to the wall, hit it and go back with (mostly) the same momentum but in the reverse direction. The change of momentum is a force- by definition again. Is there a component of the force parallel to the surface.. yes there is and it is called friction or shear force. But this force is a function of velocity as it is given by the shear stress; S=µ ∂v /∂n, where µ is viscosity, v velocity near the wall and n is normal to the surface. For a static fluids ‘v’ is zero and the force parallel to the surface becomes zero too. Noet that at zero velocity, the parallel forces do not appear because they balance each other from different directions. Motion disturbs this balance- breaks the symmetry of these forces if you like. The viscosity law expresses this empirically. And as pointed by others also, the normal forces are not symmetric wherein the walls represents half the fluid and takes the resultant of the forces on the other half to create such a balance.
Pressure (normal stress) is force per unit area- this is the definition. Since force and area are vectors, the definition chooses the component that is normal to the surface. The force comes from the change in momentum of the agitated fluid particles as they hit the wall (in reality they never touch.. just deflected back by static forces). They come with a certain momentum normal to the wall, hit it and go back with (mostly) the same momentum but in the reverse direction. The change of momentum is a force- by definition again. Is there a component of the force parallel to the surface.. yes there is and it is called friction or shear force. But this force is a function of velocity as it is given by the shear stress; S=µ ∂v /∂n, where µ is viscosity, v velocity near the wall and n is normal to the surface. For a static fluids ‘v’ is zero and the force parallel to the surface becomes zero too. Noet that at zero velocity, the parallel forces do not appear because they balance each other from different directions. Motion disturbs this balance- breaks the symmetry of these forces if you like. The viscosity law expresses this empirically. And as pointed by others also, the normal forces are not symmetric wherein the walls represents half the fluid and takes the resultant of the forces on the other half to create such a balance.
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Pressure is a scalar so it doesn't have a direction unlike Force or Area which are vectors, having a direction and magnitude. So think of it in this way:
Normally,
Force = Pressure X Area
But if we considered a vectorial representation,
Force vector = Pressure Scalar X Area vector
So the area vector's direction is the direction in which the force is going to act which would be normal to a given surface.
At any point in the fluid the force due to pressure acts in all directions but once the fluid is in contact with a surface, the force is restricted to a particular direction. This obviously works for a fluid at rest. If the fluid moves with respect to the surface, an additional force due to the shear stress acts along the surface.
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