Armed Services Vocational Aptitude Battery (ASVAB) Mechanical Comprehension (MC) Syllabus

State Archimedes' Principle of buoyancy.

The buoyant force on a submerged or floating object equals the weight of the fluid it displaces.

What determines whether an object floats or sinks in a fluid?

Density. If the object is less dense than the fluid it floats; if denser, it sinks. Equivalently, it floats if buoyant force ≥ its weight.

How does fluid pressure change with depth?

Pressure increases with depth because of the weight of the fluid above. Deeper points experience greater pressure.

What is a lever, and what are its three parts?

A lever is a rigid bar that pivots about a fixed point. Its three parts are the fulcrum (pivot), the effort (applied force), and the load (resistance).

Describe a first-class lever and give an example.

In a first-class lever the fulcrum is between the effort and the load. Examples: a seesaw, a pair of scissors, or a crowbar (pry bar).

Describe a second-class lever and give an example.

In a second-class lever the load is between the fulcrum and the effort. Examples: a wheelbarrow or a nutcracker.

Describe a third-class lever and give an example.

In a third-class lever the effort is between the fulcrum and the load. Examples: tweezers, a fishing rod, or the human forearm.

What is the formula for the mechanical advantage of a lever?

MA = effort arm length / load arm length = (distance from fulcrum to effort) / (distance from fulcrum to load).

What does mechanical advantage (MA) tell you about a machine?

It is the factor by which a machine multiplies the input force: MA = output (load) force / input (effort) force. An MA greater than 1 means the machine increases force.

On a balanced lever, if the effort is twice as far from the fulcrum as the load, what is the force advantage?

A mechanical advantage of 2: a given effort can lift twice that load (but the effort must move twice the distance).

How does a single fixed pulley help you, and what is its mechanical advantage?

A single fixed pulley changes the direction of the force (e.g., pull down to lift up) but does not multiply force, so its mechanical advantage is 1.

What is the mechanical advantage of a single movable pulley?

2, because two rope segments share the load. The effort needed is half the load, but you must pull twice the length of rope.

How do you find the mechanical advantage of a block-and-tackle pulley system?

Count the number of rope segments that directly support the movable block (the load). That count equals the mechanical advantage.

When two meshed gears turn, how do their rotation directions compare?

Two directly meshed (externally meshed) gears rotate in opposite directions. Adding an idler gear between them makes the driver and driven gears turn the same direction.

If a small gear drives a large gear, what happens to speed and torque?

The large gear turns slower than the small gear but with more torque. Gearing down trades speed for force.

How is the gear ratio of two gears calculated?

Gear ratio = number of teeth on driven gear / number of teeth on driver gear (or driven gear speed compared to driver). It equals the inverse ratio of their rotational speeds.

If a driver gear has 10 teeth and the driven gear has 30 teeth, how fast does the driven gear turn relative to the driver?

The driven gear turns at one-third the speed of the driver (10/30), making one revolution for every three of the driver, with three times the torque.

What is an inclined plane and why is it useful?

An inclined plane is a flat sloped surface (a ramp) used to raise a load. It lets you lift a load using less force over a longer distance instead of lifting it straight up.

What is the mechanical advantage of an inclined plane?

MA = length of the slope (ramp) / vertical height (rise). A longer, gentler ramp gives a greater mechanical advantage.

How is a wedge related to an inclined plane?

A wedge is essentially two inclined planes back to back. It converts a force applied to its blunt end into sideways splitting forces. Examples: an axe, knife, or chisel.

How does a screw work as a simple machine?

A screw is an inclined plane wrapped around a cylinder. Its threads convert rotational motion into linear motion and multiply force; closer thread spacing (more threads per inch) gives greater mechanical advantage.

How does a wheel and axle multiply force?

A force applied to the large-radius wheel produces a greater force at the small-radius axle (or vice versa). MA = radius of wheel / radius of axle.

In a belt drive, how do the two pulleys rotate when the belt is not crossed?

Both pulleys rotate in the same direction. (A crossed belt makes them rotate in opposite directions.)

In a belt-and-pulley system, how do pulley size and rotational speed relate?

They are inversely related: a smaller pulley turns faster and a larger pulley turns slower. A small drive pulley connected to a large pulley reduces speed.

What is the difference between tension and compression in a structure?

Tension is a stretching/pulling force that tends to lengthen a material; compression is a squeezing/pushing force that tends to shorten it.

What is the engineering definition of stress, and how is it calculated?

Stress is the internal force per unit cross-sectional area within a material: Stress = Force / Area. It indicates how concentrated the load is in the material.

What is strain in materials?

Strain is the deformation (change in length divided by original length) that a material undergoes in response to applied stress. It is a unitless ratio.

Why are triangular shapes commonly used in trusses and structural supports?

The triangle is the most rigid shape; its fixed angles cannot deform without changing side lengths, so it distributes loads efficiently and resists collapse.

Distinguish between an elastic deformation and a plastic deformation of a material.

Elastic deformation is temporary: the material returns to its original shape when the load is removed. Plastic deformation is permanent: the material stays deformed after the load is removed.

What is the difference between a ductile material and a brittle material?

A ductile material (e.g., copper, mild steel) can stretch and deform significantly before breaking. A brittle material (e.g., glass, cast iron) breaks suddenly with little deformation.

What does a cam do in a mechanical device?

A cam is a rotating shaped wheel that converts rotary motion into reciprocating (back-and-forth) or up-and-down motion of a follower, as in an engine's valve mechanism.