Physics: Mechanics, Fields and Modern Physics
Physics in JEE Advanced spans mechanics, electrodynamics, optics, thermodynamics, and modern physics, but the paper rarely asks single-formula recall. Problems combine two or three chapters - for example, a charged particle moving in crossed fields inside a capacitor, or a thermodynamic cycle coupled to a spring - and numerical-answer questions require a final value to the requested precision.
Core rules and concepts
- Newton’s second law is applied in inertial frames; in an accelerating frame add a pseudo-force −m a_frame at the centre of mass.
- Work–energy theorem: W_all forces = ΔK. Static friction can do zero work in rolling, so mechanical energy may still be conserved.
- Conservation of linear momentum holds when net external force is zero along that direction; angular momentum about a point holds when external torque about that point is zero.
- Gauss’s law is useful only with symmetry; otherwise use superposition or Poisson’s equation.
- For lenses and mirrors, fix one sign convention and keep it: Cartesian with distances measured from the pole, real-positive for mirrors, and consistent focal signs.
Worked example
A uniform solid disc of mass m, radius R, rolls without slipping down a fixed incline of angle θ. Find its centre-of-mass acceleration.
Take torques about the instantaneous contact point P. Gravity’s torque is mgR sin θ. The moment of inertia about P is I_P = (1/2)mR² + mR² = (3/2)mR². Thus α = mgR sin θ / [(3/2)mR²] = 2g sin θ / (3R). Rolling gives a = αR, so a = 2g sin θ / 3. The friction force is static and does no work; it is not mg sin θ.
Traps that cost marks
- Applying a = g sin θ to a rolling body and ignoring rotational inertia.
- Using Gauss’s law for a finite non-symmetric charge distribution.
- Mixing lens/mirror sign conventions mid-problem.
- Dropping the pseudo-force in a lift or accelerating wedge.
- Reporting a numerical answer without checking units or significant figures.