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

• I
• annamal
In summary: in summary, water does not have a shearing force, but if there is a shearing force on the sides of the glass, then the glass would break.
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?

annamal said:
Is it true that water has no shearing force but it can be sheared?
Water has no internal ability to resist shear.
annamal said:
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
Baluncore said:
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?

annamal said:
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
Baluncore said:
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?

annamal said:
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
Baluncore said:
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.

annamal said:
why doesn't water exert a downward force on the glass
? Why would it?

phinds said:
? Why would it?
? How would it?

russ_watters said:
? How would it?
An even better question.

annamal said:
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.

annamal said:
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
Chestermiller said:
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.

russ_watters and Chestermiller
annamal said:
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.

Lord Jestocost, vanhees71, hutchphd and 1 other person

## 1. What does it mean that water has no shear force?

Shear force refers to the force that is applied parallel to a surface, causing it to slide or deform. In the case of water, it means that there is no force acting on the water molecules that would cause them to slide or deform.

## 2. How is it possible for water to be sheared if it has no shear force?

While water itself does not have a shear force, it can still be sheared by external forces. For example, if a strong wind blows over the surface of a body of water, it can cause the water to move in a shearing motion.

## 3. Can water be sheared in a controlled manner?

Yes, water can be sheared in a controlled manner through techniques such as stirring, mixing, or using specialized equipment like a rheometer. These methods apply a specific amount of force to the water in a controlled manner, causing it to shear in a predictable way.

## 4. What are the practical applications of understanding water's shear behavior?

Understanding how water can be sheared is important in various industries, such as in the production of food and beverages, cosmetics, and pharmaceuticals. It is also relevant in fields like hydrology and oceanography, where the movement of water is a crucial factor.

## 5. Can water be sheared indefinitely?

No, water cannot be sheared indefinitely. As the shearing force continues to act on the water, it will eventually reach a point where the molecules can no longer resist and will start to break apart. This is known as the yield point, and beyond this point, the water will start to flow or deform more easily.

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