Water has no shear force but it can be sheared?

  • #1
annamal
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Is it true that water has no shearing force but it can be sheared? If so, would the sides of a glass holding water shear the water?
 

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  • #2
Baluncore
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Is it true that water has no shearing force but it can be sheared?
Water has no internal ability to resist shear.
If so, would the sides of a glass holding water shear the water?
In what sense?
If you empty a bucket of water onto an upright empty glass, the walls of the glass will shear the water into two parts, one part filling the glass, the other pouring onto the table.
 
  • #3
annamal
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Water has no internal ability to resist shear.

In what sense?
If you empty a bucket of water onto an upright empty glass, the walls of the glass will shear the water into two parts, one part filling the glass, the other pouring onto the table.
With a static glass of water, are the sides of the glass shearing the water as a reaction force from gravity?
 
  • #4
Baluncore
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With a static glass of water, are the sides of the glass shearing the water as a reaction force from gravity?
No. If the situation is static, flow is not occurring, so there is no shear force in the liquid.
The walls oppose the hydrostatic pressure of the liquid on the walls of the container.
 
  • #5
annamal
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No. If the situation is static, flow is not occurring, so there is no shear force in the liquid.
The walls oppose the hydrostatic pressure of the liquid on the walls of the container.
Why doesn't gravity create a shearing force at the sides of the glass?
 
  • #6
Baluncore
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Why doesn't gravity create a shearing force at the sides of the glass?
It did, but once the water moved against the side of the glass, in laminar flow, the water settled into a static position. You described the situation as static.

Shear forces would be parallel to the wall, while hydrostatic pressure due to gravity is normal to the wall and so does not result in movement. If shear forces had remained in the liquid, then the liquid would be unable to resist that shear, and so the liquid would flow further towards a static equilibrium with a level upper surface.

It seems you do not understand the term "shear" or the definition of a "liquid".
https://en.wikipedia.org/wiki/Shear_stress#Shear_stress_in_fluids
https://en.wikipedia.org/wiki/Liquid
https://en.wikipedia.org/wiki/Laminar_flow
 
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  • #7
annamal
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It did, but once the water moved against the side of the glass, in laminar flow, the water settled into a static position. You described the situation as static.

Shear forces would be parallel to the wall, while hydrostatic pressure due to gravity is normal to the wall and so does not result in movement. If shear forces had remained in the liquid, then the liquid would be unable to resist that shear, and so the liquid would flow further towards a static equilibrium with a level upper surface.

It seems you do not understand the term "shear" or the definition of a "liquid".
https://en.wikipedia.org/wiki/Shear_stress#Shear_stress_in_fluids
https://en.wikipedia.org/wiki/Liquid
https://en.wikipedia.org/wiki/Laminar_flow
No, I understand the definition of shear force and laminar flow. What I mean is at the sides of a glass, why doesn't water exert a downward force on the glass, and the glass exerts an equal and opposite upwards force on the water.
 
  • #11
Baluncore
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What I mean is at the sides of a glass, why doesn't water exert a downward force on the glass, and the glass exerts an equal and opposite upwards force on the water.
Because the surface between the glass and the static water is lubricated with a liquid that cannot transmit a shear parallel to the wall.
 
  • #12
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No, I understand the definition of shear force and laminar flow. What I mean is at the sides of a glass, why doesn't water exert a downward force on the glass, and the glass exerts an equal and opposite upwards force on the water.
It is the bottom of the glass that is exerting the force that balances the weight of the water.
 
  • #13
Baluncore
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It is the bottom of the glass that is exerting the force that balances the weight of the water.
If the glass was tapered, then the hydrostatic pressure (normal to the wall), would have a minor vertical component due to the column of the water above. But there can be no shear component to the hydrostatic pressure as pressure is always normal to the wall.
 
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  • #14
Arjan82
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Is it true that water has no shearing force but it can be sheared? If so, would the sides of a glass holding water shear the water?
This is somewhat inaccurate. The actual definition of a fluid, like water, is that it cannot withstand a shearing force without continuous deformation, or according to Fox and McDonald: "a fluid is a substance that deforms continuously under the application of a shear (tangential) stress no matter how small the shear stress may be". So a fluid that is deforming/flowing can have an internal shearing force (due to viscosity). A fluid that is static however cannot have a shearing force. All of the forces that keep water in its place in a glass are normal forces or pressure forces.
 
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