Physics - Mechanics-Led Numerical Reasoning
What the paper tests
Physics in the IOE entrance rewards clean application of a small set of laws under time pressure. Mechanics (kinematics, Newton's laws, work–energy, momentum, circular motion, gravitation, rotational motion) dominates, followed by heat and thermodynamics, simple harmonic motion and waves, electricity and magnetism, optics, and modern physics. Most items are one-formula numericals in SI units, with a minority of conceptual statements about direction, sign and conservation.
Core rules
- Kinematics: v = u + at, s = ut + ½at², v² = u² + 2as - valid only for constant acceleration.
- Energy–momentum: momentum is conserved in every collision; kinetic energy only in elastic ones.
- Circular motion and SHM: a = v²/r; for a spring–mass system T = 2π√(m/k) and for a simple pendulum T = 2π√(l/g).
- Waves: v = fλ; for a stretched string v = √(T/μ).
- Circuits: Ohm's law V = IR, series resistances add, parallel resistances add as reciprocals, Kirchhoff's junction and loop rules for networks.
- Optics: 1/f = 1/v − 1/u with the Cartesian sign convention; mirror and lens powers add for thin lenses in contact.
- Modern physics: photoelectric equation hf = φ + Kmax; N = N₀e^(−λt) for radioactive decay.
Worked example
A 2 kg body moving at 10 m/s strikes a stationary 3 kg body and they stick together. Momentum conservation gives (2)(10) = (2 + 3)v, so v = 4 m/s. Initial kinetic energy = ½(2)(10²) = 100 J; final = ½(5)(4²) = 40 J; hence 60 J is lost as heat and deformation - exactly what a perfectly inelastic collision predicts.
Traps that cost marks
- Forgetting to convert grams to kilograms, cm to m, or minutes to seconds.
- Sign errors in the mirror/lens convention (real-is-positive versus Cartesian).
- Assuming kinetic energy is conserved in a collision that is only described as bodies sticking.
- Using g = 10 m/s² when the options were built with 9.8 m/s², or the reverse.
- Confusing angular frequency ω with frequency f (ω = 2πf).