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ASVAB Practice Test · Mechanical Comprehension

ASVAB Practice Test: Mechanical Principles Concepts

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Overview

This chapter covers how simple machines and basic forces work. You will review levers, pulleys, gears, inclined planes, and pressure in liquids and gases. Most questions ask which way something moves or how much force is needed.

A simple machine makes work easier by trading force for distance. With a lever, a longer effort arm means less effort is needed: effort times effort arm equals load times load arm. A pulley system with more rope sections holding the load needs less pulling force, but you must pull more rope.

Gears pass motion from one shaft to another. Two meshed gears turn in opposite directions. A small gear driving a large gear makes the large gear turn slower but with more turning force, and a large gear driving a small one makes the small gear turn faster.

An inclined plane, or ramp, lets you raise a load with less force by moving it a longer distance. A screw is an inclined plane wrapped around a post. Pressure is force divided by area, and in a hydraulic system a small force on a small piston can make a large force on a large piston.

Key concepts

  • Lever

    A bar that turns on a fixed point called the fulcrum. Effort x effort arm = load x load arm. A longer effort arm means less effort is needed.

    Memory tipPush farther from the fulcrum to lift more easily.

  • Mechanical advantage

    How many times a machine multiplies your force: load / effort. A higher number means less effort, but you move a longer distance.

    Memory tipYou trade distance for force.

  • Pulleys

    A fixed pulley only changes the direction of pull. A movable pulley reduces the force needed. In a block and tackle, the mechanical advantage is about the number of rope sections holding the load.

    Memory tipCount the ropes holding the load.

  • Gears

    Meshed gears turn in opposite directions. A small gear driving a large gear gives slower speed and more turning force. A large gear driving a small gear gives higher speed.

    Memory tipEvery other gear in a chain turns the same way.

  • Inclined plane and screw

    A ramp lets you raise a load with less force over a longer distance. A longer, gentler ramp needs less force. A screw is a ramp wrapped around a post.

    Memory tipLonger ramp, easier push.

  • Pressure and hydraulics

    Pressure = force / area. In a closed liquid system, pressure is passed equally, so a small force on a small piston makes a larger force on a larger piston.

    Memory tipBigger piston, bigger force.

Often tested

  • Effort x effort arm = load x load arm.
  • Two meshed gears turn in opposite directions.
  • Less force always means more distance.
  • A fixed pulley only changes the direction of force.
  • Pressure = force / area.

Easy to confuse: Small gear vs. large gear (when meshed)

GearSpeedTurning force
Smaller gearTurns fasterLess
Larger gearTurns slowerMore

3 sample questions

Verified practice questions from this unit.

Practice questionReviewedMechanical Comprehension › Mechanical Principles★★★★☆

Question 1. A 100-pound load sits 2 feet from the fulcrum of a lever. The effort is applied 4 feet from the fulcrum on the other side. How much effort is needed to balance the load?

  1. ① 200 pounds
  2. ② 50 pounds
  3. ③ 25 pounds
  4. ④ 100 pounds
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ② 50 pounds

Key point: Lever: effort x effort arm = load x load arm

Use the lever rule.

  1. Effort x 4 ft = 100 lb x 2 ft
  2. Effort x 4 = 200
  3. Effort = 200 / 4 = 50 lb
  4. Check: 50 x 4 = 200 and 100 x 2 = 200. Balanced.

Wrong choices

  • (A) 200 pounds: 200 comes from multiplying by the ratio the wrong way. A longer effort arm means less effort.
  • (C) 25 pounds: 25 pounds would need an effort arm 8 feet long.
  • (D) 100 pounds: 100 pounds would balance only if both arms were the same length.
Practice questionReviewedMechanical Comprehension › Mechanical Principles★★★★☆

Question 2. Gear A meshes directly with gear B. If gear A turns clockwise, which way does gear B turn?

  1. ① It does not turn
  2. ② Clockwise
  3. ③ Back and forth
  4. ④ Counterclockwise
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ④ Counterclockwise

Key point: Meshed gears turn in opposite directions

When two gears mesh, the teeth of one push the teeth of the other in the opposite direction. So if one turns clockwise, the other turns counterclockwise.

Wrong choices

  • (A) It does not turn: Meshed teeth push each other, so B must turn when A turns.
  • (B) Clockwise: Two meshed gears always turn in opposite directions, so B cannot also turn clockwise.
  • (C) Back and forth: Steady turning of A gives steady turning of B, not back-and-forth motion.
Practice questionReviewedMechanical Comprehension › Mechanical Principles★★★★☆

Question 3. What is the main advantage of using a single fixed pulley to lift a load?

  1. ① It cuts the needed force in half
  2. ② It lifts the load twice as fast
  3. ③ It changes the direction of the pulling force
  4. ④ It makes the load lighter
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Answer: ③ It changes the direction of the pulling force

Key point: Fixed pulley: changes direction, MA = 1

A single fixed pulley is attached to a support and does not move. It lets you pull down to lift a load up, which is easier, but the force needed is the same as the load's weight.

Wrong choices

  • (A) It cuts the needed force in half: Halving the force needs a movable pulley; a single fixed pulley has a mechanical advantage of 1.
  • (B) It lifts the load twice as fast: With a mechanical advantage of 1, the load moves at the same speed as the rope is pulled.
  • (D) It makes the load lighter: No pulley changes the actual weight of the load.
Practice 5 questions from this unit →