Which of the following is NOT a Fundamental S.I unit?

Analysis

The question asks us to identify which of the given units is not a fundamental S.I. unit. In physics, units are categorized into two types: Fundamental (Base) units and Derived units.

Concept Explanation

• Fundamental Units: These are the basic units of measurement that do not depend on any other units. There are seven base S.I. units: • Metre (length) • Kilogram (mass) • Second (time) • Ampere (electric current) • Kelvin (temperature) • Mole (amount of substance) • Candela (luminous intensity)
• Derived Units: These are units formed by combining fundamental units through multiplication or division. For example, the unit for force (Newton) is derived from mass, length, and time ($F = ma$, so $1\text{ N} = 1\text{ kg}\cdot\text{m/s}^2$).

Step-by-Step Solution

1. Evaluate Option A (Metre): This is the base unit for length. It is a fundamental unit.
2. Evaluate Option B (Ampere): This is the base unit for electric current. It is a fundamental unit.
3. Evaluate Option C (Newton): This is the unit for force. As shown by the formula $F = ma$, it is defined as $\text{kg}\cdot\text{m/s}^2$. Since it is composed of other base units, it is a derived unit, not a fundamental one.
4. Evaluate Option D (Second): This is the base unit for time. It is a fundamental unit.

Conclusion

Since the Newton is a derived unit (a combination of kilograms, metres, and seconds), it does not belong to the list of fundamental S.I. units.

Exam Tip: To remember the seven fundamental units easily, use the mnemonic "L-M-T-A-K-M-C" (Length, Mass, Time, Ampere, Kelvin, Mole, Candela). If a unit is not on this list, it is almost certainly a derived unit!