Physics
Physics in NBE Sciences covers mechanics, properties of matter, heat, optics, electricity, and sound, with emphasis on applied pressure, flow, and optical systems relevant to clinical equipment. Expect formula-based numericals with straightforward arithmetic.
Core rules
- Newton's second law: force equals mass times acceleration. Work is force times displacement along the force; kinetic energy is one-half mv squared.
- Stress is force per area; strain is change in length over original length. Elastic modulus is stress divided by strain.
- Poiseuille flow: volume flow rate is proportional to the fourth power of radius and inversely proportional to length and viscosity. Halving radius reduces flow sixteenfold.
- Snell's law: n1 sin(theta1) = n2 sin(theta2). Light bends toward the normal when entering a denser medium.
- Thin lens: 1/f = 1/v - 1/u, using Cartesian sign convention; f is positive for converging lenses.
- Ohm's law: V = IR; resistors in series add, and parallel conductance adds.
Worked example
An object is 30 cm from a converging lens of focal length 10 cm. Using 1/f = 1/v - 1/u with u = -30 cm and f = +10 cm: 1/10 = 1/v + 1/30, so 1/v = 1/10 - 1/30 = 1/15. Therefore v = +15 cm, a real image on the opposite side. Magnification is v/u = 15/(-30) = -0.5, so the image is inverted and half-size.
Traps
- Keep sign conventions consistent; mixing them is the most common error.
- Convert cm to m and grams to kg before substituting into SI formulas.
- Gauge pressure excludes atmospheric pressure; absolute pressure includes it.