FCTC Applying Mechanical Reasoning 5 — Questions and Answers
Question 1: A steel I-beam is used rather than a solid rectangular beam of the same weight. What is the main advantage of the I-beam shape?
- It is easier to weld
- It resists bending more efficiently by placing material where stress is highest (Correct answer)
- It is less expensive to manufacture
- It conducts heat better
Correct answer: It resists bending more efficiently by placing material where stress is highest
The I-beam shape concentrates material in the flanges where bending stress is greatest, maximizing stiffness per unit weight.
Question 2: A valve is described as 'normally closed.' What does this mean in a pneumatic circuit?
- The valve requires manual force to open it
- Air flows freely through the valve until a signal closes it
- No air flows through the valve until an actuating signal opens it (Correct answer)
- The valve only works in one direction
Correct answer: No air flows through the valve until an actuating signal opens it
A normally closed (NC) valve blocks flow in its default rest state and only allows flow when an actuating signal is applied.
Question 3: A rotating shaft develops a crack perpendicular to its axis. What type of stress is most likely causing this failure?
- Compressive stress
- Shear stress
- Torsional stress
- Bending fatigue stress (Correct answer)
Correct answer: Bending fatigue stress
Cracks perpendicular to the shaft axis on a rotating shaft are characteristic of cyclic bending fatigue, which creates tension-compression cycles.
Question 4: Heat is transferred through a metal rod from a hot end to a cool end. What mode of heat transfer is this?
- Convection
- Radiation
- Conduction (Correct answer)
- Evaporation
Correct answer: Conduction
Heat traveling through a solid material from molecule to molecule is conduction — the dominant heat transfer mode in metals.
Question 5: A pump raises water 40 feet and delivers 100 gallons per minute. One gallon of water weighs about 8.34 lb. Approximately how much horsepower is required (1 HP = 33,000 ft-lb/min)?
- 0.5 HP
- 1 HP (Correct answer)
- 1.7 HP
- 3.4 HP
Correct answer: 1 HP
Power = (100 gal/min × 8.34 lb/gal × 40 ft) ÷ 33,000 = 33,360 ÷ 33,000 ≈ 1 HP.
Question 6: A nut is seized on a bolt. A mechanic applies penetrating oil and waits, then uses a longer breaker bar. The longer bar helps because it:
- Increases the rotational speed applied to the nut
- Increases the torque applied with the same force (Correct answer)
- Reduces friction between nut and bolt threads
- Increases the clamping force on the nut
Correct answer: Increases the torque applied with the same force
A longer bar increases the lever arm distance, so the same applied force produces greater torque (Torque = Force × Distance).
Question 7: A differential gear system in a vehicle allows the two drive wheels to rotate at different speeds. When is this most important?
- When accelerating on a straight road
- When driving at highway speed
- When turning a corner (Correct answer)
- When braking hard
Correct answer: When turning a corner
During cornering, the outer wheel must travel a longer arc than the inner wheel, requiring different rotational speeds — the differential allows this.
FCTC Applying Mechanical Reasoning 5
A steel I-beam is used rather than a solid rectangular beam of the same weight.
What is the main advantage of the I-beam shape?