Mechanical Comprehension
15 questions · 22 minutes
What's covered
Physical principles and simple machines, usually presented with a diagram. Heavy in the line scores for mechanical and engineering jobs.
The six simple machines
Lever, wheel and axle, pulley, inclined plane, wedge, screw. Every complex machine is a combination of these.
Levers
First class has the fulcrum in the middle (seesaw, crowbar). Second class has the load in the middle (wheelbarrow). Third class has the effort in the middle (tweezers, your forearm). Balance when Force₁ × distance₁ = Force₂ × distance₂ — so 100 lb at 2 ft from the fulcrum balances 50 lb at 4 ft.
Mechanical advantage
Output force ÷ input force. Machines trade distance for force; they never create energy. A ramp's MA is length ÷ height. A pulley system's MA is the number of rope sections supporting the load — a single fixed pulley has an MA of 1 and only changes direction.
Gears and belts
Meshed gears turn in opposite directions; an idler gear between them restores the original direction without changing the speed ratio. A small gear driving a large gear produces slower rotation with more torque; the reverse gives speed at the cost of torque. Belt drives keep direction the same unless the belt is crossed.
Torque, work, and power
Torque = force × perpendicular distance from the pivot, which is why a longer wrench loosens a stubborn bolt. Work = force × distance. Power = work ÷ time — power is the one that involves time.
Fluids
Pressure = force ÷ area, so the same force on a smaller area produces higher pressure. Pascal's principle: pressure applied to a confined fluid is transmitted equally, which is how a small hydraulic piston moves a large load. Liquid pressure depends on depth and density, not on the container's shape. An object floats when its density is less than the fluid's.
Friction and efficiency
Friction always opposes motion and converts energy into heat. It's why no machine reaches 100% efficiency — a machine that's 75% efficient is losing the remaining 25% mostly to friction and heat.
Springs and structures
Springs in parallel are stiffer; in series, softer. Force = spring constant × compression.
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