Specific Force

As per the momentum equation, the algebraic sum of all external forces acting in a given direction on a fluid mass is equal to the time rate of change of linear momentum of the fluid mass in that direction.

Also, dividing the equation by y, we get,

Note:

Critical flow is characterized by the following condition:

• Specific energy is minimum for a given discharge.
• Specific force is minimum for a given discharge.
• Velocity head is equal to half the hydraulic depth.
• Froude number is equal to unity.
• The discharge is maximum for a given specific energy.

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