NFST Mechanical Aptitude 3 — Questions and Answers
Question 1: A centrifugal pump is running but delivering no water. Which of the following is the most likely cause?
- The discharge pressure is too high
- The pump is air-locked (not primed) (Correct answer)
- The impeller is spinning too fast
- The discharge valve is fully open
Correct answer: The pump is air-locked (not primed)
Centrifugal pumps cannot pump air; if not primed with water, they spin without moving fluid, a condition called air-lock or vapor lock.
Question 2: On a fire apparatus, what is the purpose of a pressure governor?
- To limit the vehicle's road speed
- To maintain a preset discharge pressure automatically (Correct answer)
- To increase engine RPM during pumping
- To measure total water volume discharged
Correct answer: To maintain a preset discharge pressure automatically
A pressure governor automatically adjusts engine throttle to maintain a steady, preset discharge pressure despite changes in demand.
Question 3: A wrench handle is 12 inches long and 30 lbs of force is applied. What is the torque applied to the bolt?
- 2.5 ft-lbs
- 30 ft-lbs (Correct answer)
- 42 ft-lbs
- 360 ft-lbs
Correct answer: 30 ft-lbs
Torque = Force × Distance = 30 lbs × 1 ft (12 inches) = 30 ft-lbs.
Question 4: Which of the following materials has the highest thermal conductivity and would transfer heat most rapidly?
- Wood
- Fiberglass
- Copper (Correct answer)
- Rubber
Correct answer: Copper
Copper is an excellent thermal conductor and transfers heat far more rapidly than wood, fiberglass, or rubber.
Question 5: A spring scale reads 50 lbs when an object is weighed in air, but only 40 lbs when submerged in water. What is the buoyant force acting on the object?
- 5 lbs
- 10 lbs (Correct answer)
- 40 lbs
- 90 lbs
Correct answer: 10 lbs
Buoyant force equals the difference between the weight in air and the apparent weight in water: 50 - 40 = 10 lbs.
Question 6: When two gears mesh together, in which direction does the driven gear rotate relative to the driver gear?
- The same direction
- The opposite direction (Correct answer)
- It depends on the number of teeth
- It depends on the size of the gears
Correct answer: The opposite direction
When two gears mesh directly, they always rotate in opposite directions; an idler gear is needed to make them rotate in the same direction.
Question 7: What is the primary purpose of a check valve in a fire hose or pump system?
- To reduce water pressure
- To allow flow in one direction only (Correct answer)
- To increase flow velocity
- To filter debris from the water supply
Correct answer: To allow flow in one direction only
A check valve permits fluid to flow in one direction only, preventing backflow that could contaminate the water supply or damage equipment.
NFST Mechanical Aptitude 3
A centrifugal pump is running but delivering no water.
Which of the following is the most likely cause?