The correct option is **C. Force**. ### Detailed Explanation In the International System of Units (SI), quantities are classified into two main categories: **base quantities** and **derived quantities**. 1. **Base Quantities**: These are the fundamental physical quantities from which other quantities can be derived. The seven base quantities in the SI system are: - Length (meter, m) - Mass (kilogram, kg) - Time (second, s) - Electric current (ampere, A) - Thermodynamic temperature (kelvin, K) - Amount of substance (mole, mol) - Luminous intensity (candela, cd) 2. **Derived Quantities**: These are quantities that are derived from the base quantities through mathematical relationships. They are expressed in terms of the base quantities. Examples of derived quantities include: - Area (length squared, m²) - Volume (length cubed, m³) - Speed (length per time, m/s) - Acceleration (length per time squared, m/s²) - Force (mass times acceleration, kg·m/s², which is also known as a Newton, N) ### Why Force is a Derived Quantity Force is defined by Newton's second law of motion, which states that the force acting on an object is equal to the mass of that object multiplied by its acceleration (F = m·a). Here: - Mass (m) is a base quantity measured in kilograms (kg). - Acceleration (a) is derived from the base quantities of length and time, specifically m/s². Thus, force combines these base quantities (mass and acceleration) to create a derived quantity. The unit of force, the Newton (N), is defined as 1 N = 1 kg·m/s². ### Why the Other Options are Incorrect - **A. Length**: Length is a base quantity in the SI system. It is measured in meters (m) and does not depend on any other quantities for its definition. - **B. Mass**: Mass is also a base quantity in the SI system. It is measured in kilograms (kg) and is fundamental to the measurement of matter. - **D. Time**: Time is another base quantity in the SI system, measured in seconds (s). Like length and mass, it is fundamental and does not derive from other quantities. ### Summary of Key Points - **Base Quantities**: Length, mass, and time are fundamental and do not derive from other quantities. - **Derived Quantities**: Force is derived from the base quantities of mass and acceleration. - **Units**: Force is measured in Newtons (N), which is a combination of base units (kg·m/s²). - **Understanding Relationships**: Knowing how derived quantities relate to base quantities is crucial for solving physics problems. ### Revision Summary - **Base quantities** in SI: Length, mass, time, electric current, temperature, amount of substance, luminous intensity. - **Derived quantity**: Force (F = m·a) is derived from mass and acceleration. - **Units**: Force is measured in Newtons (N), which equals kg·m/s². - **Key Concept**: Recognize the difference between base and derived quantities for better understanding of physics principles.