Physics: Model Selection and Quantitative Setup
Physics on the EJU tests mechanics, thermodynamics, waves, electricity and magnetism, and atomic physics. Most items ask you to choose a model, set up equations, and interpret a graph or numerical result.
Core rules and concepts
- Mechanics: resolve forces on a free-body diagram; use ΣF=ma componentwise. Energy and momentum conservation are often faster than kinematics when time is unknown. Circular motion needs net centripetal force mv²/r; SHM periods are T=2π√(m/k) and T=2π√(l/g).
- Thermodynamics: first law ΔU=Q+W, where W is work done on the gas; ideal gas pV=nRT.
- Waves: v=fλ; refraction obeys n₁sinθ₁=n₂sinθ₂; thin lenses use 1/f=1/v+1/u with one sign convention.
- Electricity: Coulomb F=kq₁q₂/r²; Kirchhoff's laws; magnetic force F=BIl sinθ; induced emf ε=-dΦ/dt.
- Atomic: K_max=hf-W.
Worked example
A 2.0 kg block starts from rest and slides 5.0 m down a frictionless incline. Find its speed at the bottom. Mechanical energy is conserved: mgh=½mv², so v=√(2gh)=√(2×9.8×5.0)=9.9 m/s. The normal force is perpendicular to motion and does no work; mass cancels, so the result is independent of mass.
Traps that cost marks
Using W=+pΔV when the gas does work (the first law then needs ΔU=Q−W); mixing degrees and radians in SHM; forgetting that centripetal force is a net force, not an extra force; applying lens formulas without a fixed sign convention; rounding g to 10 when options require 9.8.