Physics · Units And Measurements · NEET
| Definition | Stands on its own; not built from other quantities | Built by combining base quantities (x or /) |
| How many | Exactly 7 fixed by SI convention | Very large number (unlimited) |
| Examples | Length, mass, time, current, temperature, mole, luminous intensity | Speed, area, force, energy, density, pressure |
| Units | Base units (metre, kilogram, second...) | Derived units (m/s, newton, pascal...) |
| Formula | No defining formula | Always has a defining formula (e.g. F = ma) |
Speed is a derived quantity. It is defined as distance divided by time, so speed = length / time. Because it is built from two base quantities (length and time), it cannot be fundamental. The same logic makes velocity, acceleration, force, and energy all derived.
Mass is one of the 7 base quantities. It is a basic property of matter and is not defined using any other quantity. Weight is the pull of gravity on that mass: weight = mass x acceleration due to gravity (W = mg). Since weight is built from mass and acceleration, it is a derived quantity, and its unit is the newton, not the kilogram.
Both, and they pair up one to one. There are 7 fundamental (base) quantities and each has its own base unit in the SI system: length (metre), mass (kilogram), time (second), electric current (ampere), temperature (kelvin), amount of substance (mole), luminous intensity (candela). The quantity is the physical thing you measure; the unit is the standard you compare it against.
No. Force is derived. From Newton's second law, force = mass x acceleration, and acceleration itself is length / time squared. So force is built entirely from base quantities (mass, length, time). Its SI unit, the newton, is a derived unit equal to kg m s^-2.
Ask one question: can I write it using any other quantity through multiplication or division? If yes, it is derived (like area = length x length, or density = mass / volume). If it stands alone and is not built from anything else, it is one of the 7 fundamental quantities. In NCERT, the 7 base quantities are fixed, so anything not on that list of 7 is derived.
If force [F], acceleration [A] and time [T] are chosen as the fundamental physical quantities, find the dimensions of energy.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Length (metre, m), mass (kilogram, kg), time (second, s), electric current (ampere, A), thermodynamic temperature (kelvin, K), amount of substance (mole, mol), and luminous intensity (candela, cd). These 7 are the base of the entire SI system.
Area (length x length), volume (length x length x length), speed (length / time), density (mass / volume), and force (mass x acceleration). Each is built by combining base quantities through multiplication or division.
Yes, in principle. Which quantities we call fundamental is a choice by convention, not a law of nature. The 2021 NEET question shows this: if you choose force, acceleration, and time as fundamental, then mass and energy become derived. NCERT and NEET use the standard SI choice of 7 base quantities unless a question states otherwise.
No. Plane angle (radian) and solid angle (steradian) are treated as dimensionless supplementary units in SI. They are ratios of two lengths or areas, so they are not among the 7 fundamental quantities.
It is the foundation of dimensional analysis, which is a high-yield NEET topic. If you know which quantities are base, you can write dimensional formulae, check equations, and derive relations. Mistaking a derived quantity for a fundamental one leads to wrong dimensional formulae and lost marks.
