Mechanical Comprehension

The Mechanical Comprehension subtest of the ASVAB measures your understanding of mechanical and physical principles. This knowledge is crucial for military jobs related to engineering, aircraft maintenance, vehicle repair, and other technical fields.

Test Format

  • CAT-ASVAB: 16 questions in 20 minutes.
  • Paper ASVAB: 25 questions in 19 minutes.

Understanding forces, motion, simple machines, and mechanical systems will significantly enhance your test performance.

1. Basic Mechanical Principles

Mechanics is the study of how objects move and interact with forces. You don’t need to be an expert in engineering, but you should understand how forces act on objects, how energy is transferred, and how simple machines work to make tasks easier.

Military jobs in engineering, aviation, vehicle repair, and weapons systems require mechanical knowledge to operate and maintain equipment effectively.

2. Principles of Mechanical Devices

Mechanical Advantage

Machines help us multiply force, making it easier to perform work. This concept is called mechanical advantage (MA), which is the ratio of output force to input force.

MA=Output ForceInput ForceMA = \frac{\text{Output Force}}{\text{Input Force}}MA=Input ForceOutput Force​

A higher MA means less effort is needed to complete a task.

Simple and Compound Machines

Machines can be simple (one mechanism) or compound (combining multiple simple machines).

  1. Levers – Reduce effort by pivoting on a fulcrum.
  2. Inclined Planes – Reduce the force needed to lift objects.
  3. Wedges – Redirect force to split or hold materials.
  4. Screws – Convert rotational force into linear force.
  5. Wheels and Axles – Reduce friction and make movement easier.
  6. Pulleys – Change the direction of force; multiple pulleys increase MA.
  7. Gears – Transfer and modify force through rotation.

Structural Support

Structures must distribute weight evenly to prevent collapse. The placement of supports affects how forces are shared.

Properties of Materials

  • Heat conduction – Metals conduct heat well; insulators do not.
  • Flexibility – Some materials bend without breaking.
  • Malleability – Materials like metal can be shaped without cracking.

3. Forces and Motion

Mass

Mass is the amount of matter in an object. It does not change based on location (unlike weight, which depends on gravity).

Force

Force is a push or pull exerted on an object. Common forces include:

  • Gravity – Pulls objects toward Earth.
  • Tension – A force exerted through a rope or string.
  • Friction – Opposes motion.
  • Recoil – Force pushing an object backward after an impact.
  • Magnetism – Attractive or repulsive force between objects.
  • Static electricity – Attraction between charged particles.

Newton’s Laws of Motion

  1. First Law (Inertia): An object in motion stays in motion unless acted upon by an external force.
  2. Second Law (F = ma): Force equals mass times acceleration.
  3. Third Law: For every action, there is an equal and opposite reaction.

Equilibrium

Equilibrium occurs when all forces acting on an object balance out, keeping the object stable.

Applied Force

An external force applied to an object can cause it to move, stop, or change direction.

4. Gravity and Friction

Gravity

Gravity is the force of attraction between two masses. On Earth, it gives objects weight and causes them to fall toward the ground.

W=mgW = mgW=mg

Where:

  • W = Weight (Newtons)
  • m = Mass (kg)
  • g = Gravity (9.8 m/s² on Earth)

Friction

Friction opposes motion and comes in different types:

  • Static Friction: Prevents motion from starting.
  • Sliding Friction: Opposes objects already in motion.
  • Rolling Friction: Resistance when objects roll (e.g., tires).
  • Fluid Friction: Resistance in liquids or air.

Elastic Recoil

Objects like springs and rubber bands return to their original shape after being stretched or compressed.

5. Fluid Dynamics

Air Pressure

  • Air has weight, creating atmospheric pressure.
  • Pressure decreases at higher altitudes due to less air density.

Water Pressure

  • Pressure increases with depth because of the weight of the water above.

Hydraulic Systems

Hydraulic systems use incompressible fluids to transfer force, allowing small inputs to produce large outputs (e.g., hydraulic jacks, brakes).

F1/A1=F2/A2F_1 / A_1 = F_2 / A_2F1​/A1​=F2​/A2​

Filling and Emptying Tanks

  • The flow rate of a liquid depends on pressure and pipe size.
  • Knowing inflow and outflow rates helps calculate how long a tank takes to fill or empty.

6. Torque and Rotational Motion

Torque

Torque is a twisting force that causes rotation, measured in Newton-meters (Nm).

Torque=Force×Lever Arm Distance\text{Torque} = \text{Force} \times \text{Lever Arm Distance}Torque=Force×Lever Arm Distance

Example: A wrench applies more torque when force is applied farther from the pivot point.

Gears and Pulleys

  • Gears control speed and torque in machines.
  • Pulleys help lift heavy loads with less force.

Centrifugal and Centripetal Forces

  • Centrifugal Force: Outward force in a spinning object.
  • Centripetal Force: Inward force keeping an object moving in a circle.

7. Work, Energy, and Power

Work

Work is done when force moves an object.

W=F×dW = F \times dW=F×d

Where:

  • W = Work (Joules)
  • F = Force (Newtons)
  • d = Distance (Meters)

Energy

  • Kinetic Energy: Energy of motion.
  • Potential Energy: Stored energy due to position.

KE=12mv2KE = \frac{1}{2} m v^2KE=21​mv2 PE=mghPE = mghPE=mgh

Power

Power is the rate of doing work.

P=WtP = \frac{W}{t}P=tW​

Measured in:

  • Watts (W)
  • Horsepower (HP) = 746 Watts

8. Mechanical Motion and Machines

Mechanical Advantage Calculation

MA=Effort DistanceLoad DistanceMA = \frac{\text{Effort Distance}}{\text{Load Distance}}MA=Load DistanceEffort Distance​

Lever Types

  1. Class 1: Fulcrum between effort/load (e.g., seesaw).
  2. Class 2: Load between fulcrum/effort (e.g., wheelbarrow).
  3. Class 3: Effort between fulcrum/load (e.g., tweezers).

Pulleys

  • Fixed Pulley: Changes direction of force.
  • Movable Pulley: Reduces effort required.
  • Block and Tackle: Uses multiple pulleys for maximum MA.

Inclined Planes and Wedges

  • Inclined Plane: Reduces effort by spreading work over distance.
  • Wedge: Applies force over a small area (e.g., axe).

Screws and Wheel & Axle

  • Screw: Converts rotation into linear motion.
  • Wheel & Axle: Reduces friction in movement.

9. Applications of Mechanical Principles

  • Hydraulic Jacks: Use fluid pressure to lift loads.
  • Vises and Clamps: Use screws to hold objects in place.
  • Barometers: Measure atmospheric pressure.

10. Test-Taking Tips

  • Eliminate non-mechanical answers first.
  • Balance effects of force changes (e.g., increasing lever length increases MA).
  • Understand basic physics concepts like Newton’s Laws, torque, and work.

By mastering these principles, you’ll significantly improve your ASVAB Mechanical Comprehension score and be prepared for technical military careers.