Final answer:
The pressure gradient normal to a horizontal curved streamline is positive. This is because a positive pressure gradient is necessary to provide the centripetal force needed for the fluid to follow a curved path.
Step-by-step explanation:
The question asks about the pressure gradient normal to a horizontal curved streamline. This concept is a fundamental principle in fluid dynamics, an area of physics that describes the behavior of liquids and gases in motion. According to Bernoulli's principle, the pressure in a moving fluid decreases as the fluid's velocity increases. If we consider a horizontal curved streamline, the pressure gradient normal to the streamline will not be zero because the pressure needs to be higher on the outside of the curve to provide the centripetal force that keeps the fluid moving in a curved path.
Therefore, the correct answer to what the pressure gradient normal to a horizontal curved streamline is: 2) It is positive. This positive pressure gradient is what provides the necessary force for the fluid particles to maintain their curved path, especially when the radius of curvature is considered. The pressure increases with the increasing curvature of the streamline (i.e., smaller radius implies larger pressure gradient).