18 exam-style questions on Physics · Fluid dynamics, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Ten Physics · Fluid dynamics questions with answers
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. In steady streamline flow, two streamlines
A
cross where the speed is greatest
B
never cross each other
C
meet wherever the pipe narrows
D
cross whenever the flow changes direction
Show answer
Answer: B. A streamline shows the direction of the fluid velocity at each point. If two crossed, the fluid at that point would move in two directions at once, which is impossible in steady flow.
2. Smooth (streamline) flow of a fluid along a pipe becomes turbulent when
A
its speed rises above a certain critical value
B
its speed falls below a certain critical value
C
its viscosity is made much greater
D
the pipe has the same cross-section all along
Show answer
Answer: A. Streamline flow is possible only up to a critical speed; above it the flow becomes irregular, with eddies and swirls (turbulent). A greater viscosity helps to keep the flow streamline.
3. Two raindrops of the same liquid fall through air with terminal velocities in the ratio 1 : 4. Taking the terminal velocity to be proportional to r2 (Stokes' law), the ratio of their masses is
A
1 : 8
B
1 : 4
C
1 : 16
D
1 : 64
Show answer
Answer: A. vt ∝ r2, so the radii are in the ratio 1 : 2. Mass ∝ volume ∝ r3, which gives 1 : 8. 1 : 64 cubes the ratio of the velocities instead of the ratio of the radii.
4. In Bernoulli's equation, P + ½ρv2 + ρgh = constant, the term ½ρv2 represents the
A
kinetic energy per unit mass
B
kinetic energy per unit volume
C
potential energy per unit volume
D
pressure energy per unit mass
Show answer
Answer: B. ½ρv2 = ½(m/V)v2, the kinetic energy of unit volume of the fluid (J m−3, the same unit as Pa). ρgh is the potential energy per unit volume.
5. The same small sphere falls through two liquids, X and Y, whose viscosities are η and 2η. Using vt = 2gr2ρ/9η, the ratio of its terminal velocities in X and Y is
A
1 : 2
B
2 : 1
C
4 : 1
D
1 : 4
Show answer
Answer: B. vt ∝ 1/η, so in the liquid with twice the viscosity the terminal velocity is half as large: vX : vY = 2 : 1. 1 : 2 wrongly takes vt to be proportional to η.
6. Water flows through a horizontal pipe that narrows from cross-sectional area 2A to A. In the wide part its speed is 2.0 m s−1. Taking the density of water as 1000 kg m−3, the pressure in the wide part is greater than that in the narrow part by
A
6.0 kPa
B
8.0 kPa
C
2.0 kPa
D
12 kPa
Show answer
Answer: A. Continuity: v2 = 2 × 2.0 = 4.0 m s−1. Bernoulli: P1 − P2 = ½ρ(v22 − v12) = ½ × 1000 × (16 − 4.0) = 6.0 × 103 Pa. 2.0 kPa squares the difference of the speeds, and 8.0 kPa ignores the speed in the wide part.
7. In the carburettor of a petrol engine, air rushes through a narrow throat where a fine fuel jet opens. Fuel is drawn into the air stream because, at the throat, the air has
A
a low speed and a high pressure
B
a high speed and a low pressure
C
a high speed and a high pressure
D
a low speed and a low pressure
Show answer
Answer: B. By continuity the air speeds up in the narrow throat, and by Bernoulli's equation its pressure falls there. The greater pressure on the fuel in the float chamber then pushes fuel out of the jet into the air stream.
8. Blood flows through the aorta, of cross-sectional area 3.0 cm2, at 30 cm s−1. The total cross-sectional area of the capillaries it feeds is 900 cm2. The average speed of the blood in the capillaries is
A
0.10 cm s−1
B
90 cm s−1
C
10 cm s−1
D
0.010 cm s−1
Show answer
Answer: A. A1v1 = A2v2: 3.0 × 30 = 900 × v, so v = 90/900 = 0.10 cm s−1. The huge total area of the capillaries makes the blood flow through them slowly, which gives time for exchange of materials.
9. The stream of water falling from a tap becomes narrower as it falls. This is because
A
surface tension pulls the water inwards more as it falls
B
its speed increases, so its cross-section decreases
C
the air pressure around the stream squeezes it inwards
D
its speed decreases, so its cross-section decreases
Show answer
Answer: B. The volume flow rate Av is the same at every level of the stream. Gravity increases v as the water falls, so the area A must decrease.
10. Wind blows at 20 m s−1 across a flat roof of area 50 m2, while the air inside the house is still. Taking the density of air as 1.3 kg m−3, the upward force on the roof is about
A
2.6 × 104 N
B
260 N
C
1.3 × 104 N
D
650 N
Show answer
Answer: C. By Bernoulli's equation the pressure above the roof is lower by ½ρv2 = ½ × 1.3 × 202 = 260 Pa. Force = pressure difference × area = 260 × 50 = 1.3 × 104 N. 260 N is the pressure difference, not the force.
18, each with the answer and a short explanation, across 2 topics: Viscosity and drag, Continuity and Bernoulli. Every chapter test draws ten of them, so you can take it many times.
Is the MDCAT Physics chapter 5 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
