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 that are defined independently. In the SI system, there are seven base quantities: - 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 not defined independently but are instead calculated using the base quantities. **Force** is a classic example of a derived quantity. It is defined by Newton's second law of motion, which states that force is the product of mass and acceleration. The formula for force is given by: \[ F = m \cdot a \] where: - \( F \) is the force measured in newtons (N), - \( m \) is the mass measured in kilograms (kg), - \( a \) is the acceleration measured in meters per second squared (m/s²). In SI units, the newton (N) can be expressed in terms of base units: \[ 1 \, \text{N} = 1 \, \text{kg} \cdot \text{m/s}^2 \] This shows that force is derived from the base quantities of mass and length (and time, through acceleration). ### 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 defined independently of other quantities. - **D. Time**: Time is another base quantity in the SI system. It is measured in seconds (s) and is fundamental to the measurement of various physical phenomena. ### Common Pitfalls - Students often confuse derived quantities with base quantities. Remember that derived quantities are formed from base quantities through multiplication, division, or other mathematical operations. - It’s important to remember the definitions of the base quantities and how they relate to derived quantities. For example, knowing that force is derived from mass and acceleration can help clarify why it is not a base quantity. ### Revision Summary - **Base quantities** in SI include length, mass, time, electric current, temperature, amount of substance, and luminous intensity. - **Derived quantities** are formed from base quantities; force is derived from mass and acceleration. - The formula for force is \( F = m \cdot a \), and it is measured in newtons (N). - Understanding the distinction between base and derived quantities is crucial for mastering physics concepts.