Physics: Laws, Signs, and Numerical Discipline
Physics in NEET UG tests whether you can map a short stem to a law and then execute the algebra without a sign error. Mechanics, electrostatics, current electricity, optics, thermodynamics and modern physics dominate; most items reduce to a definition, a conservation principle or a standard formula.
Core rules
- SI units and dimensions are a first check: every valid equation must be dimensionally consistent. Dimensional analysis cannot supply dimensionless constants.
- For uniform acceleration, use v = u + at, s = ut + ½at² and v² = u² + 2as. Fix one positive direction before substituting.
- Newton's second law is F_net = ma; the work-energy theorem is W_net = ΔK. Mechanical energy is conserved only when non-conservative work is zero.
- In electrostatics, Coulomb force and field are vectors; potential is a scalar. Superposition applies to both, but directions matter only for vectors.
- In ray optics, apply the Cartesian sign convention consistently; lens and mirror formulas are not interchangeable.
Worked example
A car starts from rest and accelerates uniformly at 2 m/s² for 5 s. Find its final speed and distance. v = u + at = 0 + 2(5) = 10 m/s. s = ut + ½at² = 0 + ½(2)(5²) = 25 m. Check: v² = u² + 2as gives 100 = 0 + 2(2)(25), so the result is consistent.
Traps
Treating g as positive while upward is positive; mixing average and instantaneous velocity; using the lens formula for a mirror; dropping units when lengths are given in cm; assuming conservation of energy when friction acts.