Physics
Physics
The physics section checks mechanics, waves, electricity and magnetism, and thermal physics through short calculations and graph reading. Most marks come from choosing the right conservation law and keeping units consistent.
Core rules
- Resolve vectors into perpendicular components; never add magnitudes unless they are parallel.
- Use conservation of energy when forces are conservative; use impulse–momentum when collision time matters.
- For projectile motion, horizontal and vertical motions are independent; vertical acceleration is g downward.
- In series circuits the current is the same and voltage divides; in parallel circuits voltage is the same and current divides.
- Wave speed v=fλ; refraction changes speed and wavelength, not frequency.
- Thermodynamics: ΔU=Q−W under a stated sign convention.
Worked example
A ball is thrown horizontally at 20 m/s from a cliff 45 m high. Take g=9.8 m/s² and ignore air resistance. Vertical motion: 45=½gt², so t=√(2×45/9.8)=√9.18≈3.03 s. Horizontal velocity stays 20 m/s, so range x=20×3.03≈61 m. Final vertical speed is gt≈29.7 m/s; resultant speed is √(20²+29.7²)≈35.8 m/s. The reasoning separates the axes; treating the initial velocity as having a vertical component would be wrong.
Traps that cost marks
- Mixing g=9.8 and g=10 within one question without instruction.
- Using wavelength in cm with frequency in Hz without converting to metres.
- Confusing average and instantaneous velocity.
- Adding a normal force or friction in the wrong direction.
- Reporting a scalar where direction is required.