NFST Mechanical Aptitude 5 — Questions and Answers
Question 1: A threaded bolt has 20 threads per inch. How far does the bolt advance into a nut with each full rotation?
- 0.05 inches (Correct answer)
- 0.10 inches
- 0.20 inches
- 2.0 inches
Correct answer: 0.05 inches
Pitch = 1 / threads per inch = 1/20 = 0.05 inches of linear advance per full turn.
Question 2: When a fire hose is kinked, what happens to the water flow and pressure on the discharge side of the kink?
- Flow and pressure both increase
- Flow decreases and pressure behind the kink increases (Correct answer)
- Flow increases and pressure decreases
- Flow and pressure remain unchanged
Correct answer: Flow decreases and pressure behind the kink increases
A kink acts like a restriction: flow downstream drops sharply while pressure builds up behind the kink on the supply side.
Question 3: Which of the following best explains why a larger diameter hose can deliver more water at the same pressure compared to a smaller hose?
- Larger hose has less friction loss per unit length (Correct answer)
- Larger hose has a higher wall strength
- Larger hose is more flexible
- Larger hose operates at lower pressure
Correct answer: Larger hose has less friction loss per unit length
Friction loss decreases dramatically as hose diameter increases, allowing greater volume flow at the same operating pressure.
Question 4: A reciprocating piston pump strokes 60 times per minute and displaces 0.5 gallons per stroke. What is its flow rate?
- 30 GPM (Correct answer)
- 60 GPM
- 120 GPM
- 0.5 GPM
Correct answer: 30 GPM
Flow rate = Strokes per minute × Displacement per stroke = 60 × 0.5 = 30 GPM.
Question 5: What is the purpose of a pressure-reducing valve in a multi-story building's standpipe system?
- To boost pressure on upper floors
- To prevent excessively high pressures on lower floors (Correct answer)
- To measure the total water volume used
- To prevent backflow into the city main
Correct answer: To prevent excessively high pressures on lower floors
Lower floors in tall buildings experience very high static pressure; pressure-reducing valves limit outlet pressure to safe, usable levels for firefighting.
Question 6: Which of the following describes the mechanical principle behind a screw jack used to lift a vehicle?
- A rotating wheel and axle converting torque to lift
- A rotating inclined plane (helix) converting rotational force to linear lifting force (Correct answer)
- A fixed pulley redirecting downward force upward
- A lever pivoting about a fixed fulcrum
Correct answer: A rotating inclined plane (helix) converting rotational force to linear lifting force
A screw is essentially a helical inclined plane; rotation applies force along the helix, converting torque into linear lifting force over a small distance.
Question 7: An air compressor tank is rated at 150 PSI maximum working pressure. A pressure gauge shows 140 PSI. If air continues to be pumped in, what safety device should activate next?
- The check valve closes
- The pressure relief valve opens (Correct answer)
- The unloader valve closes permanently
- The pressure switch increases motor speed
Correct answer: The pressure relief valve opens
When tank pressure reaches the set limit, the pressure relief valve opens to vent excess air, preventing a dangerous over-pressure condition.
NFST Mechanical Aptitude 5
A threaded bolt has 20 threads per inch.
How far does the bolt advance into a nut with each full rotation?