Fluid Dynamics

28 free MDCAT Physics MCQs on Fluid Dynamics (PM&DC syllabus), each with the answer, an explanation and the textbook line it comes from. Tap to check, no sign-up.

Tap an answer to check it · 28 questions

1. Question 1

   The resistance offered by different layers of a fluid to its flow is called:
   • AViscosity
   • BDensity
   • CUpthrust
   • DPressure
   Show answer and explanation

   Answer: A) Viscosity

   Honey has high viscosity; water and air have low viscosity. Source: 11 Physics Book FBISE, p. 107.

2. Question 2

   The retarding force experienced by an object moving through a fluid is called:
   • AUpthrust
   • BDrag force
   • CLift force
   • DWeight
   Show answer and explanation

   Answer: B) Drag force

   In air it is air resistance; in water, hydrodynamic drag. Source: 11 Physics Book FBISE, p. 107.

3. Question 3

   The maximum constant velocity acquired by a freely falling object in a viscous medium is its:
   • ACritical velocity
   • BEscape velocity
   • CTerminal velocity
   • DInitial velocity
   Show answer and explanation

   Answer: C) Terminal velocity

   Reached when drag balances weight. Source: 11 Physics Book FBISE, p. 107.

4. Question 4

   Flow in which every particle passing a point follows exactly the same smooth path as the earlier particles is:
   • ACompressible flow
   • BTurbulent flow
   • CRotational flow
   • DStreamline flow
   Show answer and explanation

   Answer: D) Streamline flow

   Also called steady or laminar flow; streamlines never cross. Source: 11 Physics Book FBISE, p. 107.

5. Question 5

   Irregular flow of a fluid, with whirlpool-like regions, is called:
   • ATurbulent flow
   • BStreamline flow
   • CLaminar flow
   • DSteady flow
   Show answer and explanation

   Answer: A) Turbulent flow

   It occurs at high speeds or with sudden changes in the tube. Source: 11 Physics Book FBISE, p. 107.

6. Question 6

   For a normal person, the systolic blood pressure is about:
   • A120 torr
   • B80 torr
   • C760 torr
   • D40 torr
   Show answer and explanation

   Answer: A) 120 torr

   Diastolic pressure is 75-80 torr; measured with a sphygmomanometer. Source: 11 Physics Book FBISE, p. 107.

7. Question 7

   Daniel Bernoulli derived his equation in the year:
   • A1687
   • B1738
   • C1905
   • D1662
   Show answer and explanation

   Answer: B) 1738

   Bernoulli lived 1700-1782. Source: 11 Physics Book FBISE, p. 100.

8. Question 8

   Helium-4 becomes a superfluid below a temperature of about:
   • A4.2 K
   • B0.0025 K
   • C2.17 K
   • D77 K
   Show answer and explanation

   Answer: C) 2.17 K

   Helium-3 becomes superfluid only below 0.0025 K. Source: 11 Physics Book FBISE, p. 106.

9. Question 9

   A cube of wood of side 10 cm is completely dipped in water. The upthrust on it is:
   • A1000 N
   • B98 N
   • C0.98 N
   • D9.8 N
   Show answer and explanation

   Answer: D) 9.8 N

   Upthrust = rho g V = 1000 x 9.8 x 0.001 = 9.8 N. Source: 11 Physics Book FBISE, p. 91.

10. Question 10

    Blood flows at 0.33 m/s in an aorta of radius 0.8 cm and divides equally into 30 arteries of radius 0.4 cm. The speed in each artery is about:
    • A0.044 m/s
    • B0.33 m/s
    • C1.32 m/s
    • D0.011 m/s
    Show answer and explanation

    Answer: A) 0.044 m/s

    A1 v1 = 30 A2 v2: v2 = 0.33 x 0.64 / (30 x 0.16) = 0.044 m/s; slower flow favours gas exchange. Source: 11 Physics Book FBISE, p. 99.

11. Question 11

    Water at 4 m/s and 47.1 kPa flows down a pipe to a point 3 m lower, where its speed is 3 m/s. The pressure there is about:
    • A50.6 kPa
    • B47.1 kPa
    • C29.4 kPa
    • D80 kPa
    Show answer and explanation

    Answer: D) 80 kPa

    P2 = 47.1 kPa + 1/2 x 1000 x (16 - 9) + 1000 x 9.8 x 3 = 47.1 + 3.5 + 29.4 = 80 kPa. Source: 11 Physics Book FBISE, p. 105.

12. Question 12

    Air flows at 30 m/s over the top and 20 m/s under the bottom of a wing (air density 1.29 kg/m3). The pressure difference is about:
    • A322.5 Pa
    • B129 Pa
    • C645 Pa
    • D12.9 Pa
    Show answer and explanation

    Answer: A) 322.5 Pa

    delta P = 1/2 rho (v1^2 - v2^2) = 0.645 x 500 = 322.5 Pa. Source: 11 Physics Book FBISE, p. 111.

13. Question 13

    Water flows at 2 m/s in a pipe of internal diameter 4 cm. For it to emerge at 4 m/s, the nozzle diameter should be about:
    • A2.0 cm
    • B2.8 cm
    • C1.0 cm
    • D5.6 cm
    Show answer and explanation

    Answer: B) 2.8 cm

    d2 = d1 sqrt(v1/v2) = 4 x 0.707 = 2.8 cm. Source: 11 Physics Book FBISE, p. 111.

14. Question 14

    According to Archimedes' principle, the upthrust on an immersed object is equal to the:
    • AMass of the object
    • BVolume of liquid displaced
    • CWeight of liquid displaced
    • DDensity of the liquid
    Show answer and explanation

    Answer: C) Weight of liquid displaced

    Upthrust = rho g V. Source: 11 Physics Book FBISE, p. 90.

15. Question 15

    An object in a fluid rises to the surface and floats if the upthrust on it is:
    • AEqual to its mass
    • BLess than its weight
    • CEqual to zero
    • DGreater than its weight
    Show answer and explanation

    Answer: D) Greater than its weight

    If upthrust equals weight it stays suspended; if less, it sinks. Source: 11 Physics Book FBISE, p. 89.

16. Question 16

    Upthrust arises because the pressure in a fluid:
    • AIncreases with depth
    • BDecreases with depth
    • CIs equal at all depths
    • DActs only downwards
    Show answer and explanation

    Answer: A) Increases with depth

    The upward force on the bottom of an object exceeds the downward force on its top. Source: 11 Physics Book FBISE, p. 89.

17. Question 17

    By Stokes' law, the drag on a small sphere moving through a fluid is directly proportional to its:
    • ARadius squared only
    • BRadius and velocity
    • CMass only
    • DDensity only
    Show answer and explanation

    Answer: B) Radius and velocity

    F = 6 pi eta r v; also proportional to viscosity. Source: 11 Physics Book FBISE, p. 92.

18. Question 18

    For a small sphere falling through a viscous fluid, the terminal velocity is proportional to the:
    • ACube of the radius
    • BRadius
    • CSquare of the radius
    • DSquare root of radius
    Show answer and explanation

    Answer: C) Square of the radius

    v = 2 rho g r^2 / 9 eta; it is inversely proportional to viscosity. Source: 11 Physics Book FBISE, p. 93.

19. Question 19

    A falling raindrop reaches its terminal velocity when its:
    • AWeight falls to zero
    • BDrag is greater than weight
    • CDrag falls to zero
    • DWeight is balanced by drag
    Show answer and explanation

    Answer: D) Weight is balanced by drag

    Net force and acceleration are then zero. Source: 11 Physics Book FBISE, p. 93.

20. Question 20

    Opening a parachute slows a paratrooper down because it:
    • AGives large area, large drag
    • BReduces his body weight
    • CRemoves all air resistance
    • DIncreases gravity on him
    Show answer and explanation

    Answer: A) Gives large area, large drag

    The drag first exceeds his weight, then balances it at a low terminal speed. Source: 11 Physics Book FBISE, p. 94.

21. Question 21

    In the model of ideal fluid flow, the fluid is assumed to be:
    • ACompressible and viscous
    • BIncompressible and non-viscous
    • CTurbulent and viscous
    • DCompressible and turbulent
    Show answer and explanation

    Answer: B) Incompressible and non-viscous

    The flow is also taken as laminar and irrotational. Source: 11 Physics Book FBISE, p. 96.

22. Question 22

    The equation of continuity (A v = constant) is a form of the law of conservation of:
    • AMomentum
    • BEnergy
    • CMass
    • DCharge
    Show answer and explanation

    Answer: C) Mass

    Bernoulli's equation comes from conservation of energy. Source: 11 Physics Book FBISE, p. 89.

23. Question 23

    Bernoulli's equation follows from applying to a flowing fluid the conservation of:
    • ACharge
    • BMass
    • CMomentum
    • DEnergy
    Show answer and explanation

    Answer: D) Energy

    Pressure + K.E per unit volume + P.E per unit volume = constant. Source: 11 Physics Book FBISE, p. 89.

24. Question 24

    As water falls freely from a tap, the cross-sectional area of the stream:
    • ADecreases
    • BIncreases
    • CStays constant
    • DBecomes zero
    Show answer and explanation

    Answer: A) Decreases

    Its speed increases, so A must decrease (A v = constant). Source: 11 Physics Book FBISE, p. 98.

25. Question 25

    A perfume atomizer, which emits liquid as a fine spray, works on:
    • APascal's law
    • BArchimedes' principle
    • CBernoulli's principle
    • DStokes' law
    Show answer and explanation

    Answer: C) Bernoulli's principle

    Fast air over the tube lowers the pressure, pushing liquid up. Source: 11 Physics Book FBISE, p. 101.

26. Question 26

    The statement that liquid leaves an orifice at depth h with the speed gained in free fall through h is:
    • AArchimedes' principle
    • BBernoulli's principle
    • CPascal's law
    • DTorricelli's theorem
    Show answer and explanation

    Answer: D) Torricelli's theorem

    v = sqrt(2gh). Source: 11 Physics Book FBISE, p. 108.

27. Question 27

    An aeroplane wing gets lift because the air flowing over its top:
    • AMoves faster, at low pressure
    • BMoves slower, at high pressure
    • CMoves at the same speed
    • DPushes the wing downward
    Show answer and explanation

    Answer: A) Moves faster, at low pressure

    Higher pressure below the wing gives a net upward force. Source: 11 Physics Book FBISE, p. 104.

28. Question 28

    The Venturi effect specifically occurs where a stream:
    • AExpands
    • BConstricts
    • CFlows downward
    • DIs at its centre
    Show answer and explanation

    Answer: B) Constricts

    In the narrow throat the speed rises and the pressure falls; used in carburettors and venturi meters. Source: 11 Physics Book FBISE, p. 109.

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