OAR OAR Mechanical Comprehension: Fluids 5 β Questions and Answers
Question 1: A ship displaces 500 tons of seawater. According to Archimedes' principle, the ship's weight is:
- Less than 500 tons
- Exactly 500 tons (Correct answer)
- More than 500 tons
- Cannot be determined without knowing volume
Correct answer: Exactly 500 tons
A floating object displaces fluid equal to its own weight, so the ship weighs exactly 500 tons.
Question 2: In a venturi meter, the pressure is measured at the wide inlet and the narrow throat. The flow rate is determined from the:
- Sum of the two pressures
- Temperature difference between the sections
- Pressure difference between inlet and throat (Correct answer)
- Pipe material and diameter only
Correct answer: Pressure difference between inlet and throat
A venturi meter uses the pressure drop between the wide inlet and narrow throat (via Bernoulli's equation) to calculate flow rate.
Question 3: Which of the following increases the viscosity of most liquids?
- Increasing temperature
- Decreasing temperature (Correct answer)
- Increasing flow velocity
- Reducing pipe diameter
Correct answer: Decreasing temperature
For most liquids, viscosity increases as temperature decreases because molecular cohesion strengthens at lower temperatures.
Question 4: A hydraulic cylinder with a 3-inch diameter piston must lift 1,000 lbs. What minimum pressure is needed? (Area = ΟrΒ²)
- About 53 psi
- About 71 psi
- About 141 psi
- About 212 psi (Correct answer)
Correct answer: About 212 psi
Area = Ο(1.5)Β² β 7.07 inΒ²; Pressure = Force/Area = 1000/4.71 β 212 psi using r = 1.5 in correctly.
Question 5: Laminar flow in a pipe is most likely when:
- Flow velocity is very high and pipe diameter is large
- The fluid is highly viscous and velocity is low (Correct answer)
- The pipe has many bends and rough walls
- The fluid is a gas, not a liquid
Correct answer: The fluid is highly viscous and velocity is low
Laminar flow occurs at low Reynolds numbers, favored by high viscosity, low velocity, and small pipe diameters.
Question 6: Water hammer in a piping system is caused by:
- Pipe vibration from external machinery
- Sudden stoppage or reversal of fluid flow creating a pressure shock wave (Correct answer)
- Overheating of fluid beyond boiling point
- Excessive filter blockage reducing flow
Correct answer: Sudden stoppage or reversal of fluid flow creating a pressure shock wave
Water hammer occurs when rapidly closing a valve causes fluid momentum to create a sudden high-pressure shock wave that travels through the pipe.
Question 7: If the specific gravity of mercury is 13.6, how high a column of mercury corresponds to 14.7 psi (one atmosphere)?
- About 10 inches
- About 30 inches (Correct answer)
- About 76 inches
- About 136 inches
Correct answer: About 30 inches
Standard atmospheric pressure supports approximately 29.92 inches (β 30 in) of mercury, which is why barometers use mercury.
OAR OAR Mechanical Comprehension: Fluids 5
A ship displaces 500 tons of seawater.
According to Archimedes' principle, the ship's weight is: