OAR OAR Mechanical Comprehension: Fluids 2 — Questions and Answers
Question 1: A hydraulic jack uses a small piston of area 2 in² and a large piston of area 20 in². If you apply 50 lbs to the small piston, what force does the large piston exert?
- 50 lbs
- 250 lbs
- 500 lbs (Correct answer)
- 1000 lbs
Correct answer: 500 lbs
Pascal's Law: Force ratio equals area ratio, so 50 × (20/2) = 500 lbs.
Question 2: Water flows through a horizontal pipe that narrows from 4 in² to 1 in² cross-section. How does the velocity change at the narrow section?
- Velocity decreases by half
- Velocity doubles
- Velocity quadruples (Correct answer)
- Velocity stays the same
Correct answer: Velocity quadruples
By continuity (A₁v₁ = A₂v₂), if area decreases by factor 4, velocity increases by factor 4.
Question 3: Which of the following best describes gauge pressure?
- Pressure measured relative to a perfect vacuum
- Pressure measured relative to atmospheric pressure (Correct answer)
- The pressure exerted only by liquids
- The pressure at the deepest point of a fluid
Correct answer: Pressure measured relative to atmospheric pressure
Gauge pressure is the pressure above (or below) local atmospheric pressure, not referenced to vacuum.
Question 4: A solid steel cube and a hollow steel cube of the same outer dimensions are submerged in water. Which experiences greater buoyant force?
- The solid cube
- The hollow cube
- Both experience the same buoyant force (Correct answer)
- It depends on the water temperature
Correct answer: Both experience the same buoyant force
Buoyant force equals the weight of displaced fluid, which depends only on the submerged volume — identical for both cubes.
Question 5: Oil with a specific gravity of 0.85 floats on water. If the oil layer is 10 inches thick, what pressure (in psi) does it add at the oil-water interface? (Water weighs 0.0361 psi/in)
- 0.307 psi (Correct answer)
- 0.361 psi
- 0.425 psi
- 0.850 psi
Correct answer: 0.307 psi
Pressure = specific gravity × water pressure per inch × depth = 0.85 × 0.0361 × 10 ≈ 0.307 psi.
Question 6: In Bernoulli's equation for an ideal fluid, when fluid flows faster its pressure:
- Increases proportionally
- Decreases (Correct answer)
- Remains constant
- Doubles
Correct answer: Decreases
Bernoulli's principle shows that higher fluid velocity corresponds to lower static pressure along a streamline.
Question 7: A dam holds back water 30 feet deep. At what depth below the surface is the pressure greatest?
- At the surface
- At 15 feet
- At 30 feet (Correct answer)
- Pressure is uniform throughout
Correct answer: At 30 feet
Hydrostatic pressure increases with depth (P = ρgh), so pressure is greatest at the bottom of the water column.
OAR OAR Mechanical Comprehension: Fluids 2
A hydraulic jack uses a small piston of area 2 in² and a large piston of area 20 in².
If you apply 50 lbs to the small piston, what force does the large piston exert?