How the Mechanical Comprehension subtest works
Mechanical Comprehension is the ninth subtest. The CAT-ASVAB gives you 22 minutes for 15 scored questions, nearly 90 seconds each. Many questions show a drawing and ask what happens: which way a gear turns, which pulley turns fastest, which post carries more of a load, which shelf holds the most weight, or why a pump valve opens.
The official topic list is simple machines, compound machines, mechanical motion and fluid dynamics, along with structural support and the properties of materials.
It adapts to you and does not let you go back. It is outside the AFQT, but it feeds the Air Force Mechanical composite, the Navy Mechanical and Nuclear composites, the Marine Corps Mechanical and General Technical scores, and six Army line scores, among them Combat, Field Artillery and Mechanical Maintenance.
How to solve a Mechanical Comprehension question
Four steps for any drawing:
- Name the machine or principle: gear, pulley, lever, inclined plane, spring, fluid pressure or balance of forces.
- Apply its rule. Meshed gears turn in opposite directions, and the one with more teeth turns more slowly. On a lever, force × distance from the pivot is equal on both sides. The more rope sections hold up a pulley load, the less force you need and the more rope you pull.
- Sketch on scratch paper: arrows for each direction of turn or force, and the numbers you know.
- Sense-check the answer. Machines trade force for distance; they never give you both.
Worked example
“A 6-foot plank rests on a pivot 2 feet from its left end. A 90-pound weight sits on the left end. Ignoring the weight of the plank, how much force at the right end will balance it?” A. 30 pounds, B. 45 pounds, C. 60 pounds, D. 180 pounds.
On a lever, weight × distance from the pivot must match on both sides. On the left, 90 pounds × 2 feet = 180 foot-pounds. The right end is 6 − 2 = 4 feet from the pivot, so the force needed is 180 ÷ 4 = 45 pounds, option B.
Option A divides by the whole length of the plank instead of the distance from the pivot, and option D forgets to divide at all. As a sense check, the right arm is twice as long as the left, so it needs half the force.
Tips for the real Mechanical Comprehension subtest
- Learn the rule for each simple machine: lever, pulley, wheel and axle, inclined plane, wedge, screw and gear.
- In a chain of gears, each gear reverses the direction again, so the first and third turn the same way.
- For structures, the support nearer the load carries more of it, and a brace that makes a triangle stiffens a frame.
- For fluids, pressure in a closed liquid is passed on equally in all directions, which is how hydraulic jacks multiply force, and a liquid speeds up where a pipe narrows.
- Pace at about 90 seconds a question, and sketch rather than trying to hold a gear train in your head.
Practise Mechanical Comprehension on CogniPrep
Our Mechanical Comprehension game gives you 15 adaptive questions with the real 22-minute clock, mostly from drawings of gears, belts, pulleys, levers, springs, ramps and liquids, with some structure and materials questions written in words. It starts at medium difficulty and adapts to each answer, and questions you do not reach are scored as random guesses. You get a standard score, where 50 is the average for US 18 to 23 year olds. The drawings, questions and difficulty ratings are ours, so treat the score as a guide.