In a pipe of constant cross-sectional area, if the velocity of a liquid doubles, what happens to the volumetric flow rate?

• A. It stays the same.
• B. It doubles.
• C. It halves.
• D. It increases by a factor of four.

Answer: (B)

Volumetric flow rate is the amount of liquid that passes a point per unit time, and it’s the product of cross-sectional area and velocity: Q = A × v. With a pipe of constant cross-section, the area A stays the same, so changing velocity directly scales Q. If the velocity doubles, you push the same amount of area twice as fast, so Q becomes twice its original value. The other scenarios would require different changes in velocity: staying the same would mean velocity doesn’t change Q, halving would halve Q, and increasing velocity fourfold would yield a fourfold increase in Q.