Physics: Mechanics, Fluids, and Circuits
Physics tests mechanics, work and energy, fluids and pressure, electricity, waves, optics, and thermal ideas, often in a medical or measurement context. You should be able to choose the right equation, convert units, and interpret signs and directions.
Core rules and concepts
- For constant acceleration: v = u + at, s = ut + ½at², and v² = u² + 2as. Use a sign convention consistently.
- Newton's second law: net force = mass × acceleration. Weight is mg, while mass is unchanged by location.
- Work = force × displacement in the force direction; kinetic energy = ½mv²; gravitational potential energy near Earth = mgh. Energy is conserved but may become heat.
- In a fluid at rest, gauge pressure = ρgh. This explains why an intravenous bag must be raised above the needle to overcome venous pressure.
- Ohm's law: V = IR; power = VI = I²R = V²/R. Series resistors add; parallel resistors add reciprocals.
- For a thin lens, power P = 1/f in diopters when f is in meters. A diverging lens has negative focal length and negative power.
Worked example
An IV bag of saline (density 1050 kg/m³) is 0.60 m above the needle. What gauge pressure does the column produce at the needle? Use P = ρgh = 1050 kg/m³ × 9.8 m/s² × 0.60 m. The product is 6174 Pa, or about 6.2 kPa. This is the pressure due to the height difference alone; atmospheric pressure acts on both the bag surface and the needle, so it cancels in gauge pressure. If the bag were lowered to 0.30 m, the gauge pressure would halve. The reasoning depends on using meters, not centimeters, and on recognizing that density is in kg/m³.
Traps
Mixing centimeters and meters, forgetting to square velocity in kinetic energy, and adding parallel resistors directly are common. In optics, a negative power means a diverging lens, not a weaker converging one. In circuit problems, check whether the question asks for total current or branch current.