What is the difference between static pressure and dynamic pressure?

What is the difference between static pressure and dynamic pressure?

• A. Static pressure is standing still, and dynamic pressure is water moving.
• B. Static pressure is moving; dynamic pressure is standing still.
• C. Both are the same.
• D. Dynamic is atmospheric.

Answer: (A)

The main idea is that static pressure is the pressure you have at a point in a fluid regardless of its motion, while dynamic pressure is the part of the pressure that comes from the fluid’s motion (its kinetic energy). When water is not moving, the dynamic portion is zero and you have only static pressure. When water is moving, part of the pressure is tied up as kinetic energy, which is the dynamic pressure and is proportional to 1/2 times the fluid density times the velocity squared. In irrigation terms, the total pressure head at a point is the sum of static pressure head and velocity head. That’s why the correct description is that static pressure represents pressure when the fluid is effectively standing still, and dynamic pressure represents the pressure associated with moving water. The other notions—dynamic pressure being present when water is standing still, or it being simply atmospheric, or static and dynamic being the same—don’t fit how pressure in a flowing fluid is partitioned and how velocity influences the energy inside the fluid.

The main idea is that static pressure is the pressure you have at a point in a fluid regardless of its motion, while dynamic pressure is the part of the pressure that comes from the fluid’s motion (its kinetic energy). When water is not moving, the dynamic portion is zero and you have only static pressure. When water is moving, part of the pressure is tied up as kinetic energy, which is the dynamic pressure and is proportional to 1/2 times the fluid density times the velocity squared. In irrigation terms, the total pressure head at a point is the sum of static pressure head and velocity head.

That’s why the correct description is that static pressure represents pressure when the fluid is effectively standing still, and dynamic pressure represents the pressure associated with moving water. The other notions—dynamic pressure being present when water is standing still, or it being simply atmospheric, or static and dynamic being the same—don’t fit how pressure in a flowing fluid is partitioned and how velocity influences the energy inside the fluid.