Final answer:
A U-tube manometer indicating a pressure drop signifies that the pressure on one side of the manometer is lower than on the other side. This is utilized to calculate gauge pressure based on the height difference of the liquid in the U-tube.
Step-by-step explanation:
When a u-tube manometer indicates a pressure drop, it means that the pressure on one side of the manometer is lower than the pressure on the other side. In the case of an open-tube manometer, if one side is open to the atmosphere, and the other side is connected to a gas chamber, the pressure in the gas chamber can be higher (positive gauge pressure) or lower (negative gauge pressure) than atmospheric pressure. This difference in pressure causes the manometric fluid to be at different heights in the two arms of the U-tube. The height difference, denoted as h, is used to calculate the gauge pressure (pg) as pg = hpg, where p is the density of the fluid and g is the acceleration due to gravity.