Applied Physics, Mechanics, and Technical Drawing
Scope
Units, forces, moments, stress, pressure and engineering drawing conventions.
Core rules
- SI units: force in N, moment in N·m, stress and pressure in Pa, power in W.
- Moment = force × perpendicular distance; always state the sign convention.
- Stress = force ÷ area; strain = change in length ÷ original length; Young's modulus = stress ÷ strain.
- Pressure = force ÷ area, which governs socket and orthosis fit.
- Drawing: first-angle projection is standard in India; the symbol shows a truncated cone with the circle to the left of the side view. Include hidden lines, centre lines and tolerances; check the stated scale.
Worked example
A 350 N ground reaction force acts 0.12 m behind the knee. Moment = 350 × 0.12 = 42 N·m, tending to flex the knee. A pylon of cross-section 2 cm² carries 1000 N: stress = 1000 ÷ 0.0002 m² = 5 MPa, well below the yield strength of common aluminium alloys.
Traps
Omitting units; mixing N and kgf (1 kgf ≈ 9.81 N); using third-angle projection; dimensioning to hidden lines; ignoring the drawing scale; forgetting that a moment needs a perpendicular distance.