Final answer:
When the wind stops, the pressures above and below the board equalize since there is no longer any wind to create a pressure difference. Hence, the reading on the spring scale, which reads 80 N with the wind, would be approximately 0 N when the wind stops, given the board's weight is negligible.
Step-by-step explanation:
The student's question pertains to the application of Bernoulli's principle to a scenario involving air flow, pressure differences, and force exerted on a horizontal board. When the wind moves beneath the board and the air above is stationary, the pressure beneath the board decreases, resulting in a net upward force due to the higher pressure above the board. This is why the spring scale reads 80 N when there is wind. Once the wind stops, the pressures above and below the board equalize, and the board only exerts its weight due to gravity on the spring scale. The scale reads the weight of the board in Newtons, which will be less than 80 N assuming the board has negligible weight.
Since the weight of the board is negligible and the wind is what caused the 80 N reading on the scale, the reading on the scale when the wind stops would be approximately 0 N, as the scale is only measuring the weight of the board, which is negligible.