# Water has no shear force but it can be sheared?

• I
annamal
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?

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.

• Lnewqban
annamal
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?

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.

• DaveE
annamal
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?

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

• phinds and Bystander
annamal
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.

Gold Member
2022 Award
why doesn't water exert a downward force on the glass
? Why would it?

Mentor
? Why would it?
? How would it?

Gold Member
2022 Award
? How would it?
An even better question.

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.

Mentor
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.

• Baluncore
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