Memorise the formal definition and the Q = n × u equation Commit the textbook definition: a physical quantity is one that can be measured and by which physical happenings can be expressed as laws. The product n × u = constant means that when you switch from SI to CGS, n changes inversely with u. The trap: forgetting to adjust the numerical value when the unit changes, leading to answers off by powers of 10.

Apply n₁u₁ = n₂u₂ to unit-conversion problems For any conversion, write n₁ and u₁ in the original system, then express u₁ in terms of u₂ using dimensional factors. For example, 1 N = 1 kg·m/s² = 1000 g × 100 cm / s² = 10⁵ dyne. The trap: applying the inverse relationship backwards — if the unit becomes smaller, the numerical value must become larger, not smaller.

Distinguish physical quantities from non-physical experiences NEET assertion–reason items may list comfort, anger, beauty alongside force, mass, and velocity. The test: can it be assigned a numerical magnitude and a unit? If not, it is not a physical quantity. The trap: assuming all commonly discussed attributes are physical quantities simply because they can be rated on a subjective scale.