IBEW Mechanical Principles and Pulleys 2 — Questions and Answers
Question 1: A fixed pulley changes the direction of force but not the amount. If you pull down with 50 lbs on a fixed pulley, how much weight does it lift?
- 25 lbs
- 50 lbs (Correct answer)
- 100 lbs
- 75 lbs
Correct answer: 50 lbs
A fixed pulley provides no mechanical advantage — it only redirects force. 50 lbs of pull lifts 50 lbs.
A fixed pulley is attached to a stationary point and only changes the direction of the applied force. It does NOT multiply force. The mechanical advantage (MA) = 1. Therefore, a 50-lb pull lifts exactly 50 lbs.
Question 2: A movable pulley system has 2 rope segments supporting the load. What is the mechanical advantage?
- 1
- 3
- 2 (Correct answer)
- 4
Correct answer: 2
Mechanical advantage = number of rope segments supporting the load = 2.
For pulley systems, mechanical advantage (MA) equals the number of rope segments supporting the movable pulley. With 2 segments supporting the load, MA = 2. This means you only need to apply half the force to lift the load, but you must pull twice as much rope.
Question 3: A gear with 20 teeth meshes with a gear with 40 teeth. If the smaller gear rotates at 100 RPM, what is the speed of the larger gear?
- 200 RPM
- 100 RPM
- 50 RPM (Correct answer)
- 25 RPM
Correct answer: 50 RPM
Speed ratio = teeth of driven ÷ teeth of driver. 20/40 × 100 = 50 RPM.
Gear ratio = (teeth on driver) ÷ (teeth on driven) = 20 ÷ 40 = 1/2. Speed of driven gear = 100 RPM × (20/40) = 100 × 0.5 = 50 RPM. The larger gear rotates slower. Torque is inversely proportional — the larger gear produces more torque.
Question 4: A lever has a 6-foot effort arm and a 2-foot resistance arm. What is the mechanical advantage?
- 1/3
- 2
- 3 (Correct answer)
- 4
Correct answer: 3
MA = effort arm ÷ resistance arm = 6 ÷ 2 = 3.
Mechanical advantage (MA) = effort arm ÷ resistance arm = 6 ft ÷ 2 ft = 3. This means you can lift a 300-lb load with only 100 lbs of force. The longer the effort arm relative to the resistance arm, the greater the mechanical advantage.
Question 5: Which type of pulley system provides the greatest mechanical advantage: a single fixed pulley, a single movable pulley, or a block and tackle with 4 rope segments?
- Single fixed pulley
- Single movable pulley
- Block and tackle with 4 segments (Correct answer)
- They are all equal
Correct answer: Block and tackle with 4 segments
Block and tackle with 4 rope segments: MA=4. Single movable: MA=2. Single fixed: MA=1.
Single fixed pulley: MA=1 (only redirects force). Single movable pulley: MA=2 (halves the effort needed). Block and tackle with 4 segments: MA=4 (requires only 1/4 of the load force to lift). Greater MA means less force required, but more rope must be pulled.
Question 6: A wheel and axle has a wheel radius of 12 inches and an axle radius of 3 inches. What is the mechanical advantage?
- 3
- 4 (Correct answer)
- 5
- 6
Correct answer: 4
MA = wheel radius ÷ axle radius = 12 ÷ 3 = 4.
For a wheel and axle, MA = radius of wheel ÷ radius of axle = 12 in ÷ 3 in = 4. This means applying force to the wheel requires only 1/4 of the force needed directly at the axle. Steering wheels and windlasses use this principle.
IBEW Mechanical Principles and Pulleys 2
A fixed pulley changes the direction of force but not the amount.
If you pull down with 50 lbs on a fixed pulley, how much weight does it lift?