Free Fluid Dynamics MCQs with Answers
363 Fluid Dynamics MCQs from Physics, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
Fluid flow is described through speed, pressure and density, including steady flow and the equation of continuity for conserving mass. Bernoulli's equation relates pressure, speed and height, while fluid drag and terminal velocity explain why a falling object eventually moves at constant speed when drag balances its weight.
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- A. the drag force becomes zero
- B. the upward drag and upthrust together balance the weight, so the net force is zero
- C. the body stops moving completely
- D. gravity stops acting on the body
Explanation: Drag increases with speed, so the body accelerates until the retarding forces grow to match its weight, after which the acceleration is…
Correct answer: the upward drag and upthrust together balance the weight, so the net force is zero- A. the square of the speed
- B. the speed
- C. the cube of the radius
- D. the inverse of the radius
Explanation: Stokes's law gives the drag as 6 pi eta r v, so it rises in direct proportion to both the radius and the speed, provided the flow stays…
Correct answer: the speed- A. the radius of the sphere
- B. the square of the radius of the sphere
- C. the inverse of the radius
- D. the viscosity of the liquid
Explanation: Equating the weight less upthrust to the Stokes drag gives a terminal velocity proportional to r squared and inversely proportional to…
Correct answer: the square of the radius of the sphere- A. the fluid particles move in irregular, mixing paths
- B. each fluid particle follows the same smooth path as the one before it
- C. the fluid is at rest
- D. the fluid is compressible
Explanation: In laminar or streamline flow the layers slide over one another without mixing, so the velocity at any fixed point stays constant in time…
Correct answer: each fluid particle follows the same smooth path as the one before it- A. A1 v1 equals A2 v2, so the fluid speeds up where the pipe narrows
- B. A1 v1 equals A2 v2, so the fluid slows down where the pipe narrows
- C. the pressure is the same everywhere in the pipe
- D. the density changes along the pipe
Explanation: The same volume passes every cross section each second, so where the area falls the speed must rise in proportion.
Correct answer: A1 v1 equals A2 v2, so the fluid speeds up where the pipe narrows- A. half
- B. double
- C. four times
- D. unchanged
Explanation: Area depends on the square of the radius, so halving the radius quarters the area, and by the continuity equation the speed must rise by a…
Correct answer: four times- A. conservation of mass
- B. conservation of momentum
- C. conservation of energy for a flowing fluid
- D. Newton's third law
Explanation: The equation adds pressure energy, kinetic energy and potential energy per unit volume and states that the total is the same all along a…
Correct answer: conservation of energy for a flowing fluid- A. high
- B. low
- C. unchanged
- D. zero
Explanation: Since the total energy per unit volume is fixed, any gain in kinetic energy must be paid for by a fall in pressure energy.
Correct answer: low- A. air moves faster over the curved upper surface, lowering the pressure there
- B. air moves faster under the wing
- C. the wing is lighter than air
- D. the engines push the wing upwards
Explanation: The aerofoil shape makes the air travel further and faster across the top, so by Bernoulli's principle the pressure above is lower than…
Correct answer: air moves faster over the curved upper surface, lowering the pressure there- A. the temperature rise in a constriction
- B. the pressure difference between a wide section and a narrow section
- C. the weight of the fluid
- D. the colour change of the fluid
Explanation: The fluid speeds up in the throat, so its pressure falls, and measuring that pressure drop with a manometer gives the speed through the…
Correct answer: the pressure difference between a wide section and a narrow section- 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…
Correct answer: N s per m squared- 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…
Correct answer: decreases- 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…
Correct answer: where the vessel is widest and the blood moves slowest- 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…
Correct answer: the weight of the fluid displaced by the body- 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…
Correct answer: a higher 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…
Correct answer: falls, because the larger area increases the drag at a given speed- 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…
Correct answer: stays the same along the pipe for an ideal fluid- 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…
Correct answer: the speed exceeds a critical value and the Reynolds number becomes large- 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.
Correct answer: 0.04 cubic metres per second- 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…
Correct answer: air moves faster on one side of the ball, creating a pressure difference across itFluid Dynamics MCQs: common questions
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