Physics I: Mechanics, Fluids, Waves, and Thermal Physics with Medical Applications

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The NIE Physics syllabus has eleven units totaling about 600 periods: Measurement, Mechanics, Oscillations and Waves, Thermal Physics, Gravitational Field, Electrostatic Field, Magnetic Field, Current Electricity, Electronics, Mechanical Properties of Matter, and Matter and Radiation (NIE, n.d.-e). Physics underlies medical imaging, instruments, and physiology (Hobbie & Roth, 2015). This chapter and the next summarize the units with their medical applications; the companion volume Advanced Level Physical Science develops the same syllabus in more detail.

Measurement

SI base units (m, kg, s, A, K, mol, cd) (BIPM, 2019); dimensions and dimensional analysis; errors: add absolute errors for sums and differences, fractional errors for products and quotients; vernier calipers, micrometer screw gauge, zero error; graphs and gradients.

Mechanics

Kinematics: v = u + at; s = ut + ½at²; v² = u² + 2as; projectile motion.
Newton's laws; F = ma; momentum p = mv, conserved in isolated systems; impulse = FΔt.
Work, energy, and power: W = Fs cos θ; KE = ½mv²; PE = mgh; P = Fv.
Moments and equilibrium: ΣF = 0, Στ = 0; levers in the body: the forearm, neck, and foot act as levers of different classes.
Circular motion: a = v²/r; F = mv²/r (a centrifuge separates blood components by this principle).
Friction: F ≤ μR (articular cartilage and synovial fluid give extremely low friction in joints).
Medical link: biomechanics explains posture, gait, back injury, and the forces on joints; falls and fracture risk relate to impulse and energy.

Fluids

Pressure in a fluid p = p₀ + ρgh; blood pressure is measured in mmHg (1 mmHg ≈ 133 Pa); the pressure difference between head and feet in a standing person is about ρgh.
Archimedes' principle; continuity A₁v₁ = A₂v₂ (blood speed is lowest in the capillaries, which have the largest total cross-section); Bernoulli's equation p + ½ρv² + ρgh = constant.
Viscosity: Stokes' law F = 6πηrv; Poiseuille's equation (extension) Q = πr⁴ΔP/(8ηL) shows that flow depends on the fourth power of the radius, so a small narrowing of an artery greatly reduces flow.
Surface tension: excess pressure in a bubble; the role of surfactant in alveoli (preventing collapse).

Oscillations and Waves

SHM: a = −ω²x; T = 2π/ω; resonance.
Waves: v = fλ; superposition; standing waves; interference; diffraction.
Sound: intensity level β = 10 log₁₀(I/I₀) with I₀ = 10⁻¹² W m⁻²; the Doppler effect. Medical links: the ear (hearing range about 20 Hz to 20 kHz), ultrasound imaging (frequencies of 1 to 15 MHz; reflection at tissue boundaries), Doppler ultrasound for blood flow, and lithotripsy.
Light and optics: refraction n = sin i/sin r; critical angle; lens formula 1/u + 1/v = 1/f (real-is-positive convention); power of a lens P = 1/f (in dioptres). Medical links: the eye as an optical system (cornea and lens focus light onto the retina; the near point is about 25 cm); myopia is corrected with a diverging lens and hypermetropia with a converging lens; optical fibres and endoscopes use total internal reflection; microscopes and lasers.
Electromagnetic spectrum: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma; ultraviolet causes sunburn and skin cancer.

Thermal Physics

Heat and temperature: Q = mcΔθ; Q = mL; thermal expansion; the kelvin scale.
Ideal gas: pV = nRT; kinetic theory; first law ΔU = Q − W.
Heat transfer: conduction, convection, radiation. Medical links: thermoregulation (evaporation of sweat removes latent heat; blood flow carries heat to the skin), clinical thermometers, hypothermia and heat stroke, and thermography.
Water's high specific heat keeps the body temperature stable.

Common Mistakes

Treating the pascal and mmHg as the same unit.
Mixing up the sign conventions in the first law.
Forgetting that the intensity level in decibels is logarithmic.

CHAPTER 20