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Valter Moretti
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Ideal fluids are, by definitions, continuous bodies which support only compressive stresses. It means that a portion of fluid, say, a volume with regular boundary, is such that every small area of its boundary receives a surface force (proportional to the area) from the external part of fluid, and this force is always directed towards the interior of the portion of fluid and is orthogonal to its boundary.

A portion of fluid may move only if the sum of these compressive stresses in not vanishing.

We cannot pull (ideal) fluids, we can only push them!

In your example, you are considering an approximatively cylindrical portion of fluid bounded by two lateral surfaces A1 and A2. The remaining part of the boundary is irrelevant for the computation of the work due to the stresses on this portion of fluid, since these forces are normal to the velocity of the particles of fluid.

As the forces are always compressive, the forces on the lateral surfaces must be directed along opposite directions. Since the fluid moves from the left to the right, the force on A1 must have intensity greater than the one on A2. This difference moves (pushes) the cylinder.

Ideal fluids are, by definitions, continuous bodies which support only compressive stresses. It means that a portion of fluid, say, a volume with regular boundary, is such that every small area of its boundary receives a surface force (proportional to the area) from the external part of fluid, and this force is always directed towards the interior of the portion of fluid and is orthogonal to its boundary.

A portion of fluid may move only if the sum of these compressive stresses in not vanishing.

We cannot pull fluids, we can only push them!

In your example, you are considering an approximatively cylindrical portion of fluid bounded by two lateral surfaces A1 and A2. The remaining part of the boundary is irrelevant for the computation of the work due to the stresses on this portion of fluid, since these forces are normal to the velocity of the particles of fluid.

As the forces are always compressive, the forces on the lateral surfaces must be directed along opposite directions. Since the fluid moves from the left to the right, the force on A1 must have intensity greater than the one on A2. This difference moves (pushes) the cylinder.

Ideal fluids are, by definitions, continuous bodies which support only compressive stresses. It means that a portion of fluid, say, a volume with regular boundary, is such that every small area of its boundary receives a surface force (proportional to the area) from the external part of fluid, and this force is always directed towards the interior of the portion of fluid and is orthogonal to its boundary.

A portion of fluid may move only if the sum of these compressive stresses in not vanishing.

We cannot pull (ideal) fluids, we can only push them!

In your example, you are considering an approximatively cylindrical portion of fluid bounded by two lateral surfaces A1 and A2. The remaining part of the boundary is irrelevant for the computation of the work due to the stresses on this portion of fluid, since these forces are normal to the velocity of the particles of fluid.

As the forces are always compressive, the forces on the lateral surfaces must be directed along opposite directions. Since the fluid moves from the left to the right, the force on A1 must have intensity greater than the one on A2. This difference moves (pushes) the cylinder.

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Valter Moretti
  • 78.1k
  • 8
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  • 308

Ideal fluids are, by definitions, continuous bodies which support only compressive stresses. It means that a portion of fluid, say, a volume with regular boundary, is such that every small area of its boundary receives a surface force (proportional to the area) from the external part of fluid, and this force is always directed towards the interior of the portion of fluid and is orthogonal to its boundary.

A portion of fluid may move only if the sum of these compressive stresses in not vanishing.

We cannot pull fluids, we can only push them!

In your example, you are considering an approximatively cylindrical portion of fluid bounded by two lateral surfaces A1 and A2. The remaining part of the boundary is irrelevant for the computation of the work due to the stresses on this portion of fluid, since these forces are normal to the velocity of the particles of fluid.

As the forces are always compressive, the forces on the lateral surfaces must be directed along opposite directions. Since the fluid moves from the left to the right, the force on A1 must have intensity greater than the one on A2. This difference moves (pushes) the cylinder.

Ideal fluids are, by definitions, continuous bodies which support only compressive stresses. It means that a portion of fluid, say, a volume with regular boundary, is such that every small area of its boundary receives a surface force (proportional to the area) from the external part of fluid, and this force is always directed towards the interior of the portion of fluid and is orthogonal to its boundary.

A portion of fluid may move only if the sum of these compressive stresses in not vanishing.

We cannot pull fluids, we can only push them!

In your example, you are considering an approximatively cylindrical portion of fluid bounded by two lateral surfaces A1 and A2. The remaining part of the boundary is irrelevant for the computation of the work due to the stresses on this portion of fluid, since these forces are normal to the velocity of the particles of fluid.

As the forces are always compressive, the forces on the lateral surfaces must be directed along opposite directions. Since the fluid moves from the left to the right, the force on A1 must have intensity greater than the one on A2. This difference moves the cylinder.

Ideal fluids are, by definitions, continuous bodies which support only compressive stresses. It means that a portion of fluid, say, a volume with regular boundary, is such that every small area of its boundary receives a surface force (proportional to the area) from the external part of fluid, and this force is always directed towards the interior of the portion of fluid and is orthogonal to its boundary.

A portion of fluid may move only if the sum of these compressive stresses in not vanishing.

We cannot pull fluids, we can only push them!

In your example, you are considering an approximatively cylindrical portion of fluid bounded by two lateral surfaces A1 and A2. The remaining part of the boundary is irrelevant for the computation of the work due to the stresses on this portion of fluid, since these forces are normal to the velocity of the particles of fluid.

As the forces are always compressive, the forces on the lateral surfaces must be directed along opposite directions. Since the fluid moves from the left to the right, the force on A1 must have intensity greater than the one on A2. This difference moves (pushes) the cylinder.

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Valter Moretti
  • 78.1k
  • 8
  • 169
  • 308

Ideal fluids are, by definitions, continuous bodies which support only compressive stresses. It means that a portion of fluid, say, a volume with regular boundary, is such that every small area of its boundary receives a surface force (proportional to the area) from the external part of fluid, and this force is always directed towards the interior of the portion of fluid and is orthogonal to its boundary.

A portion of fluid may move only if the sum of these compressive stresses in not vanishing.

We cannot pull fluids, we can only push them!

In your example, you are considering an approximatively cylindrical portion of fluid bounded by two lateral surfaces A1 and A2. The remaining part of the boundary is irrelevant for the computation of the work due to the stresses on this portion of fluid, since these forces are normal to the velocity of the particles of fluid. 

As the forces are always compressive, the forces on the lateral surfaces must be directed along opposite directions. Since the fluid moves from the left to the right, the force on A1 must have intensity greater than the one on A2. This difference moves the cylinder.

Ideal fluids are, by definitions, continuous bodies which support only compressive stresses. It means that a portion of fluid, say, a volume with regular boundary, is such that every small area of its boundary receives a surface force (proportional to the area) from the external part of fluid directed towards the interior of the portion of fluid and orthogonal to its boundary.

A portion of fluid may move only if the sum of these compressive stresses in not vanishing.

We cannot pull fluids, we can only push them!

In your example, you are considering an approximatively cylindrical portion of fluid bounded by two lateral surfaces A1 and A2. The remaining part of the boundary is irrelevant for the computation of the work due to the stresses on this portion of fluid since are normal to the velocity of the particles of fluid. As the forces are compressive, the forces on the lateral surfaces must be directed along opposite directions. Since the fluid moves from the left to the right, the force on A1 must have intensity greater than the one on A2.

Ideal fluids are, by definitions, continuous bodies which support only compressive stresses. It means that a portion of fluid, say, a volume with regular boundary, is such that every small area of its boundary receives a surface force (proportional to the area) from the external part of fluid, and this force is always directed towards the interior of the portion of fluid and is orthogonal to its boundary.

A portion of fluid may move only if the sum of these compressive stresses in not vanishing.

We cannot pull fluids, we can only push them!

In your example, you are considering an approximatively cylindrical portion of fluid bounded by two lateral surfaces A1 and A2. The remaining part of the boundary is irrelevant for the computation of the work due to the stresses on this portion of fluid, since these forces are normal to the velocity of the particles of fluid. 

As the forces are always compressive, the forces on the lateral surfaces must be directed along opposite directions. Since the fluid moves from the left to the right, the force on A1 must have intensity greater than the one on A2. This difference moves the cylinder.

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Valter Moretti
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