OAR Mechanical Comprehension: Fluids Flashcards

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?

Answer: 500 lbs

Pascal's Law: Force ratio equals area ratio, so 50 × (20/2) = 500 lbs.

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?

Answer: Velocity quadruples

By continuity (A₁v₁ = A₂v₂), if area decreases by factor 4, velocity increases by factor 4.

Which of the following best describes gauge pressure?

Answer: Pressure measured relative to atmospheric pressure

Gauge pressure is the pressure above (or below) local atmospheric pressure, not referenced to vacuum.

A solid steel cube and a hollow steel cube of the same outer dimensions are submerged in water. Which experiences greater buoyant force?

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.

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)

Answer: 0.307 psi

Pressure = specific gravity × water pressure per inch × depth = 0.85 × 0.0361 × 10 ≈ 0.307 psi.

In Bernoulli's equation for an ideal fluid, when fluid flows faster its pressure:

Answer: Decreases

Bernoulli's principle shows that higher fluid velocity corresponds to lower static pressure along a streamline.

A dam holds back water 30 feet deep. At what depth below the surface is the pressure greatest?

Answer: At 30 feet

Hydrostatic pressure increases with depth (P = ρgh), so pressure is greatest at the bottom of the water column.