OAR OAR Mechanical Comprehension: Fluids 4 — Questions and Answers
Question 1: Which of the following is an example of a non-Newtonian fluid?
- Water
- Air
- Ketchup (Correct answer)
- Gasoline
Correct answer: Ketchup
Ketchup is a shear-thinning non-Newtonian fluid — its viscosity decreases under stress (like shaking the bottle).
Question 2: A pipe carrying water branches into two equal pipes. If the flow rate in the main pipe is 8 gal/min, what is the flow rate in each branch?
- 8 gal/min
- 16 gal/min
- 4 gal/min (Correct answer)
- 2 gal/min
Correct answer: 4 gal/min
Conservation of mass requires the total flow to be preserved: 8 gal/min splits equally into 4 gal/min per branch.
Question 3: What is cavitation in a fluid system?
- Buildup of air bubbles at the pump inlet that collapse when pressure rises, causing damage (Correct answer)
- Gradual corrosion of pipe walls by acidic fluids
- Separation of oil and water in a mixed fluid
- Increase in fluid density under high pressure
Correct answer: Buildup of air bubbles at the pump inlet that collapse when pressure rises, causing damage
Cavitation occurs when local pressure drops below vapor pressure, forming bubbles that collapse violently and erode surfaces.
Question 4: A hydraulic system transmits 2,000 lbs of force through fluid. If there are no losses, the work done by the input piston equals the work done by the output piston because:
- Hydraulic fluid stores energy
- Energy is conserved — force × distance is equal on both sides (Correct answer)
- The fluid multiplies energy
- Larger pistons always move farther
Correct answer: Energy is conserved — force × distance is equal on both sides
Pascal's Law multiplies force but not energy; the output piston moves a shorter distance proportionally, preserving work (W = F × d).
Question 5: Which condition must be true for Bernoulli's equation to apply without correction?
- The fluid must be compressible
- The fluid must be viscous
- The flow must be steady and the fluid ideal (inviscid, incompressible) (Correct answer)
- The pipe must be vertical
Correct answer: The flow must be steady and the fluid ideal (inviscid, incompressible)
Bernoulli's equation in its basic form assumes steady, incompressible, inviscid (frictionless) flow along a streamline.
Question 6: A fluid exerts a force on a curved pipe bend. In which direction is the net resultant force on the bend if flow turns 90°?
- Along the original flow direction only
- Along the exit flow direction only
- Diagonally, combining both the inlet and outlet momentum directions (Correct answer)
- There is no net force on a smooth bend
Correct answer: Diagonally, combining both the inlet and outlet momentum directions
Momentum changes in both the x and y directions at a 90° bend, so the resultant reaction force acts diagonally.
Question 7: Atmospheric pressure at sea level is approximately 14.7 psi. What is the absolute pressure at the bottom of a pool that creates 5 psi of gauge pressure?
- 5 psi
- 9.7 psi
- 14.7 psi
- 19.7 psi (Correct answer)
Correct answer: 19.7 psi
Absolute pressure = atmospheric pressure + gauge pressure = 14.7 + 5 = 19.7 psi.
OAR OAR Mechanical Comprehension: Fluids 4
Which of the following is an example of a non-Newtonian fluid?