Free Fluid Dynamics MCQs with Answers

11. The SI unit of coefficient of viscosity is

• A. N s per m squared
• B. N per m squared
• C. N m
• D. m squared per s

Explanation: Viscosity is defined through the relation between shear stress and velocity gradient, giving units of newton second per square metre, equivalently the pascal second. The pascal, newton per square metre, is pressure rather than viscosity. In the older CGS system the unit was the poise, and one pascal second equals ten poise.

12. As the temperature rises, the viscosity of a liquid

• A. increases
• B. decreases
• C. stays the same
• D. becomes zero

Explanation: Heating gives the molecules enough energy to slip past one another more easily, weakening the intermolecular attractions that resist flow, which is why warm honey pours readily. Gases behave in the opposite way: their viscosity rises with temperature because it comes from molecular collisions rather than cohesion. Confusing the two cases is the usual mistake.

13. Blood pressure measured in a major artery is highest

• A. where the artery is narrowest
• B. where the blood flows fastest
• C. where the vessel is widest and the blood moves slowest
• D. at the capillaries

Explanation: Bernoulli's principle applies inside the body as well: where the cross section is large the speed is low and more of the energy remains as pressure. Where a vessel narrows, the blood accelerates and the lateral pressure on the wall falls. This is one reason an aneurysm, a widened weak spot, experiences a higher pressure and tends to grow.

14. Upthrust on a body immersed in a fluid is equal to

• A. the weight of the body
• B. the weight of the fluid displaced by the body
• C. the volume of the body
• D. the density of the fluid

Explanation: This is Archimedes's principle, and the upthrust arises because pressure increases with depth, so the upward force on the bottom of the body exceeds the downward force on the top. A body floats when the upthrust at partial immersion already equals its weight. The principle also explains why a body appears lighter when weighed in water.

15. Two spheres of the same size but different densities fall through the same viscous liquid. The denser one has

• A. the same terminal velocity
• B. a lower terminal velocity
• C. a higher terminal velocity
• D. no terminal velocity

Explanation: Terminal velocity depends on the difference between the density of the sphere and that of the fluid, so a denser sphere needs a larger drag force to balance its greater weight and must therefore fall faster to generate it. Radius and viscosity being equal, density is the only variable left. If the sphere were less dense than the liquid it would rise instead.

16. A skydiver opens a parachute. The terminal velocity

• A. falls, because the larger area increases the drag at a given speed
• B. rises, because the parachute adds weight
• C. is unchanged
• D. becomes zero immediately

Explanation: Opening the canopy multiplies the effective area and therefore the drag, so the forces balance again at a much lower speed and the diver decelerates towards the new terminal velocity. The weight is essentially unchanged, so it is the drag side of the balance that has altered. The speed never reaches zero, which is why a landing roll is still needed.

17. In streamline flow through a horizontal pipe of uniform cross section, Bernoulli's equation predicts that the pressure

• A. rises steadily along the pipe
• B. stays the same along the pipe for an ideal fluid
• C. falls to zero at the outlet
• D. depends on the colour of the pipe

Explanation: With constant area the speed is constant by continuity, and with no height change the potential term is constant too, so the pressure term must also be constant for an ideal fluid. Real fluids do lose pressure along such a pipe because viscosity dissipates energy, which is exactly the effect Bernoulli's equation leaves out. Comparing the ideal prediction with the real behaviour is how the role of viscosity is demonstrated.

18. The flow of a fluid becomes turbulent when

• A. the speed falls below a critical value
• B. the speed exceeds a critical value and the Reynolds number becomes large
• C. the pipe is perfectly smooth
• D. the fluid is very viscous

Explanation: Turbulence sets in when inertial effects dominate viscous ones, which is what a large Reynolds number expresses, and it appears as eddies and rapid mixing. A high viscosity favours laminar flow rather than turbulence, since it damps the disturbances out. Turbulent flow carries far more resistance, which matters in pipelines, aircraft design and blood vessels alike.

19. Water flows at 2 m per second through a pipe of cross sectional area 0.02 m squared. The volume flow rate is

• A. 0.04 cubic metres per second
• B. 0.01 cubic metres per second
• C. 4 cubic metres per second
• D. 0.4 cubic metres per second

Explanation: Flow rate is area multiplied by speed, so 0.02 times 2 gives 0.04 cubic metres per second, that is 40 litres every second. Dividing rather than multiplying gives the 0.01 distractor. The same product appears on both sides of the continuity equation, which is why it is constant along a pipe.

20. A spinning cricket ball swings in flight because

• A. gravity acts sideways on a spinning body
• B. the spin makes the ball lighter
• C. air moves faster on one side of the ball, creating a pressure difference across it
• D. the ball changes shape in the air

Explanation: The spinning surface drags air round with it, so the airflow is faster on one side and slower on the other, and by Bernoulli's principle the resulting pressure difference pushes the ball sideways. This sideways force on a spinning body is the Magnus effect, and it is why a swing bowler polishes one side of the ball. The same physics curves a football and lifts a golf ball.