Water flows through a horizontal pipe of cross-sectional area 10.0 cm2 at a pressure of 0.250 atm. The flow rate is 1.00 ˛ 10-3 m3/s. At a valve, the effective cross-sectional area of the pipe is reduced to 5.00 cm2. What is the pressure at the valve?

Pascal's principle: If an external pressure is applied to a confined fluid, the pressure at every point within the fluid increases by that amount.
Archimedes' principle: The buoyant force on an object immersed in a fluid is equal to the weight of the fluid displaced by that object.

Bernoulli's principle: Where the velocity of fluid is high, the pressure is low; and where the velocity is low, the pressure is high.