FCTC Mechanical Reasoning Principles Questions and Answers 3 — Questions and Answers
Question 1: A pulley system is used to raise a 300-pound victim. If the system has a mechanical advantage of 3, how much force must the rescuer apply (ignoring friction)?
- 100 lbs (Correct answer)
- 150 lbs
- 300 lbs
- 900 lbs
Correct answer: 100 lbs
Force equals load divided by mechanical advantage: 300 divided by 3 equals 100 pounds.
With a 3:1 mechanical advantage, only one-third of the load weight must be applied: 300 divided by 3 equals 100 pounds. In practice, friction in the rope and pulley system reduces efficiency slightly, so actual effort will be somewhat more than 100 pounds.
Question 2: Which simple machine does a firefighter use when sliding a victim down a stairway on a rescue sled?
- Lever
- Wheel and axle
- Inclined plane (Correct answer)
- Pulley
Correct answer: Inclined plane
An inclined plane reduces the force needed to move a load through a vertical height by spreading the work over a longer distance.
An inclined plane allows a load to be moved vertically with less force by trading increased distance for reduced force. A rescue sled on a staircase acts as an inclined plane because less pulling force is needed than would be required to lift the victim straight up.
Question 3: In a compound pulley system, if you must pull 6 feet of rope to raise a load by 1 foot, what is the mechanical advantage?
- 1
- 3
- 6 (Correct answer)
- 12
Correct answer: 6
Mechanical advantage equals effort distance divided by load distance: 6 divided by 1 equals 6.
For any simple machine, MA equals effort distance divided by load distance. Pulling 6 feet of rope to lift the load 1 foot gives MA of 6. A higher MA means less force is required, but the rope must be pulled a greater distance to move the load the same amount.
Question 4: A fire pump impeller spins faster when driven by a larger sprocket connected by a chain to a smaller sprocket on the impeller shaft. Which sprocket is the driver?
- The larger sprocket — it drives at lower RPM but speeds up the smaller driven sprocket (Correct answer)
- The smaller sprocket — it drives at higher RPM
- Both sprockets rotate at the same speed
- The impeller speed is unaffected by sprocket size
Correct answer: The larger sprocket — it drives at lower RPM but speeds up the smaller driven sprocket
A large driver sprocket connected to a small driven sprocket increases the output RPM on the driven shaft, the same principle as a favorable gear ratio.
In a chain drive, speed ratio equals driver teeth divided by driven teeth. A large driver sprocket connected to a small driven sprocket increases output RPM. The pump impeller mounted on the driven shaft spins faster than the power source, which is useful for increasing pump speed from a lower-speed engine.
Question 5: What is the main advantage of a worm gear mechanism used in some fire apparatus equipment?
- Very high speed transmission
- Large mechanical advantage combined with a self-locking ability (Correct answer)
- Low torque output
- Ability to change direction of rotation freely in both directions
Correct answer: Large mechanical advantage combined with a self-locking ability
Worm gears provide high gear reduction (large mechanical advantage) and are self-locking — the worm drives the gear wheel but the gear wheel cannot back-drive the worm.
Worm gears convert high-speed, low-torque input into low-speed, high-torque output. Their self-locking property prevents the load from back-driving the mechanism, which is important in aerial ladder rotation drives and pump governor systems where holding position under load is critical.
Question 6: A fire department come-along (ratchet lever hoist) is rated at 2,000 pounds. The device uses which mechanical principle to achieve this rating?
- Hydraulic force multiplication
- Ratchet and pawl combined with a lever arm to provide mechanical advantage (Correct answer)
- Electromagnetic braking
- Pneumatic pressure amplification
Correct answer: Ratchet and pawl combined with a lever arm to provide mechanical advantage
A come-along uses a ratchet mechanism to hold position and a lever arm to multiply input force, allowing one person to move heavy loads.
A come-along combines a ratchet-and-pawl (which prevents the load from reversing) with a handle lever arm that multiplies the operator's input force. The combination gives a 2,000-pound rated device that one firefighter can operate to move vehicles or debris in vehicle extrication and rescue scenarios.
FCTC Mechanical Reasoning Principles Questions and Answers 3
A pulley system is used to raise a 300-pound victim.
If the system has a mechanical advantage of 3, how much force must the rescuer apply (ignoring friction)?