Overview
This lecture covers core mechanical comprehension concepts for the Army Aviation SIP test, focusing on simple machines, mechanics, pressure, thermodynamics, electricity, circuits, and magnetism.
Simple Machines
- Six types: lever, pulley, wheel and axle, inclined plane, wedge, screw.
- Mechanical advantage (MA) = force output divided by force input; also input distance over output distance.
- Levers: MA = length of effort arm / length of resistance arm.
- Three classes of levers based on fulcrum, load, and effort positions.
- Example: For a lever with a 50 lb load 7 ft from fulcrum and effort arm 3 ft, need 35 lb force for equilibrium.
Pulleys & Belt Pulleys
- Pulley MA = number of supporting ropes in a movable pulley system.
- More pulleys reduce required force to lift loads (e.g., 4 pulleys need only 25 lb to lift 100 lb).
- Belt pulley direction: untwisted belt rotates pulleys the same way; twisted belt, opposite way.
Inclined Planes & Gears
- Inclined plane MA = length of ramp / height of ramp.
- Gears: MA and speed determined by number of teeth; formula R₁D₁ = R₂D₂ (revolution × diameter/teeth).
Cams & Pressure
- Cam and follower provide time-specific, repeated motion.
- Pressure = force / area; crucial in Bernoulli’s principle (high velocity = low pressure, low velocity = high pressure).
- Airplane wings: top surface has high velocity, low pressure; bottom surface has low velocity, high pressure (creates lift).
Thermodynamics: Heat Transfer
- Three types of heat transfer: conduction (contact), convection (fluid movement), radiation (EM waves).
- Good conductors: copper, silver, iron; insulators: air, wood, paper.
Materials & Density
- Bimetallic strip: two metals with different expansion rates bend when temperature changes.
- Density = mass / volume; higher altitude means lower air density and pressure.
Electricity & Circuits
- Charge: protons (+), electrons (−), neutrons (neutral); measured in coulombs.
- Current (I): flow of electrons, I = Q / t, measured in amperes; direct current (DC) vs. alternating current (AC).
- Voltage (V): electrical potential energy; Power = Current × Voltage (P = IV).
- Resistance (R): opposes electron flow, measured in ohms (Ω); Ohm’s Law: V = IR.
- Series circuit: total resistance RT = R1 + R2.
- Parallel circuit: 1/RT = 1/R1 + 1/R2; total resistance decreases.
- Fuse: a wire that melts to break a circuit when current is too high.
Magnetism
- Like poles repel; opposite poles attract.
Key Terms & Definitions
- Mechanical Advantage (MA) — The ratio of output force to input force in a machine.
- Fulcrum — The pivot point of a lever.
- Effort Arm — The part of a lever where force is applied.
- Resistance Arm — The part of a lever that opposes the effort.
- Current (I) — The flow of electric charge.
- Voltage (V) — Electrical potential difference.
- Resistance (R) — Opposition to the flow of current.
- Ohm’s Law — Relationship between voltage, current, and resistance (V = IR).
- Fuse — Protective device that melts when current is excessive.
Action Items / Next Steps
- Practice problems for each simple machine type.
- Memorize lever classes and formulas for MA.
- Download and use the SIP tutoring app for additional practice.
- Complete all assigned homework and try to achieve at least 80% on practice tests.