Physics · Units And Measurements · NEET
No. A physical quantity is the property you measure (like length, time, mass). A unit is the fixed standard amount of that quantity you compare against. Example: length is the quantity; metre is the unit. You always report a measurement as quantity = number x unit, so 5 m = 5 times the standard metre.
A bare number is meaningless in physics. If a rod is '5', is it 5 metres, 5 centimetres, or 5 feet? These are all different lengths. The unit tells the reader which standard the number is counting. For NEET, an answer written without a unit (or with the wrong unit) is treated as wrong.
It means the exact size of the standard was a human choice, not fixed by nature. We could have picked any length as the metre. But once chosen, it is fixed and internationally accepted, so everyone uses the same standard. 'Arbitrary' refers only to the original choice, not that you can keep changing it.
If each person used their own unit (say, their own foot length), a '5-foot' rod would be a different actual length for each person. A standard unit is one agreed value that never changes and is the same worldwide. This makes measurements repeatable and comparable — a metre in India equals a metre in the USA.
Yes. Length can be measured in metre, centimetre, kilometre, foot, or light-year. They are all valid units for the same quantity. When you change the unit, the numerical value changes, but the actual physical size stays the same: 1 m = 100 cm describes the same length with different numbers.
Yes, and it changes in the opposite direction. The physical quantity is fixed, so (number x unit) must stay constant. A smaller unit needs a bigger number: 2 m = 200 cm. This inverse relation, n1 u1 = n2 u2, is exactly how unit conversions and the NEET 'new unit' problems work.
The speed of light in vacuum is taken as unity. If light takes 6 min 40 s to reach the Earth from the Sun, the distance between the Sun and the Earth in the new unit is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT (Units and Measurement) defines a unit as a certain basic, arbitrarily chosen, internationally accepted reference standard used to measure a physical quantity. The result of a measurement is a number accompanied by that unit.
Fundamental (base) units are the units of the base quantities (like metre for length, kilogram for mass, second for time). Derived units are built by combining base units, like m/s for speed or kg m/s^2 (newton) for force.
Standard units make every measurement repeatable and comparable, and NEET always expects the correct SI unit with the numerical answer. Many Units and Measurement questions test whether you can convert between units or work in a redefined unit system.
It means the physical quantity is constant, so when you switch units, the product of number and unit stays the same. A smaller unit (u2) needs a larger number (n2). This single relation solves almost all unit-conversion and 'new unit' NEET problems.
A unit is a standard, not the quantity itself. The quantity (like length) is the property; the unit (like metre) is the agreed reference amount of that property against which we compare.
