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Mechanical Principles and Pulleys Flashcards

6 cards from real IBEW practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 6 Mechanical Principles and Pulleys flashcards as text

  1. An electrician is using a block and tackle pulley system to lift a 400-pound transformer. The system is configured with 4 rope segments directly supporting the load. Ignoring friction, what is the minimum effort force required to lift the transformer?

    Answer: 100 pounds

    The Ideal Mechanical Advantage (IMA) of a pulley system is equal to the number of rope segments supporting the load. In this case, the IMA is 4. The required effort is calculated by dividing the load's weight by the IMA. Therefore, Effort = 400 pounds / 4 = 100 pounds.

  2. An apprentice adds more pulleys to a block and tackle system to make it easier to lift a heavy piece of equipment. Which of the following is the primary trade-off for reducing the required effort?

    Answer: The rope must be pulled a longer distance.

    The principle of work states that Work = Force × Distance. To reduce the input force (effort) while performing the same amount of work (lifting the load a certain height), the input distance (the length of rope pulled) must be proportionally increased. For every foot the load is lifted, the apprentice must pull a length of rope equal to the system's mechanical advantage.

  3. What is the primary mechanical function of a single fixed pulley?

    Answer: To change the direction of the applied force.

    A single fixed pulley has an Ideal Mechanical Advantage (IMA) of 1, meaning it does not multiply the force. Its main purpose is to redirect the force, allowing a person to pull downwards to lift a load upwards. This is often a more convenient and ergonomic motion.

  4. Which of the following statements best describes the effect of friction in a real-world pulley system?

    Answer: Friction requires the actual effort force to be greater than the ideal effort force.

    Friction opposes motion in the pulley's axle and between the rope and the wheels. This means that to overcome friction and lift the load, the actual effort required is always higher than the ideal effort calculated in a frictionless system. This causes the Actual Mechanical Advantage (AMA) to always be less than the Ideal Mechanical Advantage (IMA).

  5. An electrician needs to tighten a large nut that requires 90 ft-lbs of torque. If the electrician can comfortably apply 30 pounds of force, what is the minimum length of the wrench needed to properly tighten the nut?

    Answer: 3 feet

    Torque is calculated by the formula: Torque = Force × Lever Arm Length. To find the required length of the wrench (the lever arm), the formula is rearranged to: Length = Torque / Force. Plugging in the values gives: Length = 90 ft-lbs / 30 lbs = 3 feet.

  6. A small gear with 15 teeth is driving a larger gear with 45 teeth. If the small gear rotates clockwise at 120 RPM, what will be the speed and direction of the larger gear?

    Answer: 40 RPM, counter-clockwise

    The gear ratio is the number of teeth on the driven gear divided by the number of teeth on the driver gear (45 / 15 = 3). The speed of the larger gear will be the speed of the smaller gear divided by the gear ratio (120 RPM / 3 = 40 RPM). When two external gears mesh, they rotate in opposite directions. Since the small gear rotates clockwise, the large gear must rotate counter-clockwise.