Question

SI unit of force is :

• A. Joule
• B. Newton
• C. Pascal
• D. Watt

Hint:

Remember that force makes things move. Think of "Newton" as the man who saw the apple fall due to a "force" (gravity), so the unit is named after him. All other options like Joule and Watt are related to energy and its usage over time.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the standard unit for the physical quantity of "Force" within the International System of Units (SI).
Step 2: Key Formula or Approach:
The definition of force comes from Newton's Second Law of Motion:
\[ \text{Force} (F) = \text{mass} (m) \times \text{acceleration} (a) \]
Step 3: Detailed Explanation:

• Definition of the Newton: The SI unit of force is the Newton (symbol: N), named after Sir Isaac Newton. As a derived unit, one Newton is defined as the force required to give a mass of one kilogram an acceleration of one meter per second squared.
  
• Dimensional Analysis: In terms of SI base units, the Newton is expressed as \( 1 \text{ N} = 1 \text{ kg} \cdot \text{m/s}^2 \).
  
• Analyzing Incorrect Units:
  - Joule (A): This is the SI unit of energy or work, defined as the work done when a force of one newton is applied over one meter (\( 1 \text{ J} = 1 \text{ N} \cdot \text{m} \)).
  - Pascal (C): This is the SI unit of pressure, defined as a force of one newton applied over an area of one square meter (\( 1 \text{ Pa} = 1 \text{ N/m}^2 \)).
  - Watt (D): This is the SI unit of power, which is the rate of energy transfer, defined as one joule per second (\( 1 \text{ W} = 1 \text{ J/s} \)).
  
• Importance in Physics: Force is a fundamental concept that describes an interaction that causes a change in an object's motion. Using the correct unit is essential for all calculations in mechanics.
  

Step 4: Final Answer:
The Newton is the correct SI unit for force, derived from the relationship between mass and acceleration.

Answer 2

Force can also be defined as the rate of change of momentum, \( F = \dfrac{d(mv)}{dt} \), and tracking the units through that definition, rather than through F = ma directly, leads to the same identification. Comparing this against each unit option below confirms the match.

1. Joule: A Joule carries units of \( \text{kg} \cdot \text{m}^2/\text{s}^2 \), the units of energy (force multiplied by an extra length), so it measures work or energy, not force itself.
2. Newton: Tracking momentum's units, mass times velocity gives \( \text{kg} \cdot \text{m/s} \), and dividing by time (seconds) for the rate of change gives \( \text{kg} \cdot \text{m/s}^2 \). That combination of base units is exactly what is named the Newton, so it matches force precisely.
3. Pascal: A Pascal carries units of \( \text{kg}/(\text{m}\cdot\text{s}^2) \), force divided by an area, which is the definition of pressure, not force on its own.
4. Watt: A Watt carries units of \( \text{kg}\cdot\text{m}^2/\text{s}^3 \), energy divided by time, which defines power, an entirely different rate-based quantity from force.

Only the Newton's unit combination matches what force actually reduces to in base SI units.

Therefore, the correct answer is Newton.