In a free nappe,
- AThe pressure below the nappe is atmospheric.Correct
- BThe pressure below the nappe is negative.
- CThe pressure above the nappe is atmospheric.
- DThe pressure above the nappe is negative.
Solution
Concept
A nappe is the sheet of water flowing over a weir or a spillway. When air freely circulates underneath this falling water sheet, it is called a free nappe.
Step-by-step solution
1. Understand what a nappe is. It is the sheet of water overflowing a weir. 2. Look at a free nappe. Air is allowed to enter freely underneath the falling water sheet. 3. Because air is present below the nappe, the pressure in that air pocket is equal to the outside atmospheric pressure. 4. Therefore, the pressure below a free nappe is atmospheric. Answer: (A) The pressure below the nappe is atmospheric.
Explanation
In a free nappe, atmospheric air has free access underneath the falling water sheet. This keeps the pressure below the nappe equal to atmospheric pressure.
Why the other options are wrong
B: Negative pressure happens only when air is trapped and removed, called a depressed nappe. C: The pressure above the nappe is also atmospheric, but the defining feature of a free nappe is the atmospheric pressure underneath. D: Pressure above the nappe is never negative in open-air overfalls.
Shortcut
Remember 'Free nappe = Free air = Atmospheric pressure'.
Common mistake
Thinking that falling water creates a vacuum or negative pressure, which only happens in a depressed or suppressed nappe when air is removed.
Correct answer: A) The pressure below the nappe is atmospheric.