Structures, Mechanisms and Control
Structures
Understand that structures carry loads and transmit forces. Know the types of load (tension, compression, bending, torsion, shear) and the members that resist them: ties in tension, struts in compression, beams in bending. Triangulation gives rigidity to frames; a square frame will rack without a diagonal brace. Know that stability depends on a wide base and a low centre of gravity. A frequent trap is confusing a tie with a strut, or claiming a material is 'strong' without saying whether it resists tension or compression.
Mechanisms
Learn the four basic motion types: rotary, linear, reciprocating and oscillating. Know how levers, linkages, cams and followers, gears, pulleys and belts, chain and sprocket, and screw threads convert motion. Understand mechanical advantage as output force divided by input force, velocity ratio as distance moved by effort divided by distance moved by load, and that no machine is perfectly efficient. Gear ratios change speed and torque in inverse proportion.
Control
Understand open-loop control (no feedback) and closed-loop control (a sensor feeds back to the controller). Know inputs (switches, thermistors, light-dependent resistors), processes (microcontrollers, logic) and outputs (motors, lamps, buzzers).
Worked example: a gear train
A motor turns at 1200 rpm with a 12-tooth gear driving a 36-tooth gear. Gear ratio is 36 divided by 12, which is 3:1. Output speed is 1200 divided by 3, so 400 rpm, and torque is multiplied by three (ignoring losses). If the driven gear were smaller, speed would rise and torque would fall.
Rule of thumb
State the motion conversion and the trade-off: you gain force or speed, never both.