This is explained by the continuity equation and Bernoulli's principle. The continuity equation states that for an incompressible fluid, the flow rate (A × v) is constant. Therefore, if the cross-sectional area (A) decreases, the velocity (v) must increase to maintain the same flow rate.

Bernoulli's principle states that for a horizontal flow, the sum of pressure energy and kinetic energy per unit volume is constant: $\mathrm{P}+\frac{1}{2}{\mathrm{\rho v}}^2=\text{constant}$. If velocity (v) increases, the kinetic energy term (12ρv2) increases. To keep the total constant, the pressure (P) must decrease.

Final answer: v will increase and P will decrease