Why are fundamental quantities only seven?
Fundamental quantities are those physical quantities that cannot be defined in terms of other physical quantities. There are seven fundamental quantities that serve as the base units for all other derived units in the International System of Units (SI). These fundamental quantities include length, mass, time, electric current, temperature, amount of substance, and luminous intensity.

Why can't mass or length be considered derived physical quantities?
- Mass: Mass is considered a fundamental quantity because it is an inherent property of matter and cannot be defined in terms of other physical quantities. It is used as the basis for deriving other quantities like force, momentum, and energy.
- Length: Length is also a fundamental quantity as it represents the extent of an object in one dimension and cannot be derived from other quantities. It serves as the basis for deriving other quantities like area, volume, and speed.
- Characteristics of fundamental quantities:
- Fundamental quantities are independent and cannot be expressed in terms of other quantities.
- They serve as the foundation for deriving all other physical quantities.
- Any change in fundamental quantities directly affects derived quantities.
In conclusion, mass and length are considered fundamental quantities because they are essential building blocks for defining other physical quantities. They cannot be expressed in terms of other quantities and play a crucial role in the systematic organization of units in physics.