Free Fluid Dynamics MCQs with Answers

21. An incompressible fluid flows through a pipe that becomes narrower in one section. The fluid speed increases in that region to maintain:

• A. Constant pressure
• B. Constant energy
• C. Constant mass flow rate
• D. Constant volume

Explanation: The same mass must pass every cross section each second, so where the area falls the speed must rise in proportion, which is the equation of continuity and is simply conservation of mass applied to a fluid. Pressure certainly does not stay constant; it falls in the constriction, which is Bernoulli's principle. This is why a thumb over a hose makes the water squirt further.

22. In a pipe of varying cross-section, as fluid enters the narrower region it exhibits:

• A. High velocity, high pressure
• B. High velocity, low pressure
• C. Low velocity, high pressure
• D. Low velocity, low pressure

Explanation: Continuity forces the speed up in the constriction, and since the total energy per unit volume is fixed, the gain in kinetic energy must be paid for by a drop in pressure energy. This pairing of fast flow with low pressure is the heart of Bernoulli's principle. A Venturi meter measures flow rate by reading exactly this pressure difference.

23. The curved shape of an airplane wing causes air to move faster over the top surface. This leads to:

• A. Greater pressure on the top
• B. Lower pressure on the top
• C. Equal pressure on both sides
• D. Zero pressure above the wing

Explanation: Faster flow means lower pressure by Bernoulli's principle, so the pressure above the wing falls below that underneath and the difference multiplied by the wing area provides the lift. The wing also deflects air downwards, and by Newton's third law that contributes lift as well, so both descriptions account for the same force. The pressure above never reaches zero.

24. When an object attains terminal velocity, its acceleration is:

• A. 9.8 m/s2 upward
• B. Zero
• C. 1 m/s2
• D. 9.8 m/s2 downward

Explanation: At terminal velocity the upward drag and upthrust exactly balance the weight, so the resultant force is zero and by Newton's second law the acceleration is zero, even though the body continues to fall at a steady speed. What has vanished is the resultant force, not any individual one. This is why a raindrop reaches the ground at a survivable speed.

25. A fluid is flowing through a tube. To undergo transition from laminar to turbulent flow its velocity must be:

• A. Slightly less than critical velocity
• B. Equal to critical velocity
• C. Greater than critical velocity
• D. Increasing gradually but less than critical velocity

Explanation: Below the critical velocity the flow stays in orderly layers, and only once that threshold is passed does the motion break up into eddies and become turbulent. The critical velocity is set by the Reynolds number, which combines density, viscosity, speed and pipe diameter. Turbulent flow dissipates far more energy, which matters in pipelines and in blood vessels alike.