Question

SI unit of weight is :

• A. Kg
• B. \(\text{Kg ms}^{-1}\)
• C. N
• D. None of these

Hint:

Weight is a force, specifically the force of gravity. The SI unit for any force is the Newton (N). Mass is the amount of matter, its SI unit is Kilogram (Kg). Do not confuse mass and weight; they are different physical quantities with different units.

Answer 1

The Correct Option is C

Concept: Weight is a type of force. Specifically, it is the gravitational force exerted on an object by a celestial body (like Earth, Moon, etc.). Step 1: Define weight Weight (\(W\)) of an object is given by the product of its mass (\(m\)) and the acceleration due to gravity (\(g\)) at its location: \[ W = mg \] Step 2: Determine the SI units of mass and acceleration due to gravity
The SI unit of mass (\(m\)) is the kilogram (Kg).
The SI unit of acceleration (\(g\), or any acceleration) is meters per second squared (\(\text{m/s}^2\) or \(\text{ms}^{-2}\)). Step 3: Determine the SI unit of force According to Newton's Second Law of Motion, force (\(F\)) is equal to mass (\(m\)) times acceleration (\(a\)): \(F = ma\). The SI unit of force is the Newton (N). One Newton is defined as the force required to accelerate a 1 kilogram mass by 1 meter per second squared. So, \(1 \ \text{N} = 1 \ \text{Kg} \cdot \text{m/s}^2\). Step 4: Relate the unit of weight to the unit of force Since weight (\(W = mg\)) is a force, its SI unit must be the same as the SI unit of force. Unit of weight = Unit of mass × Unit of acceleration due to gravity Unit of weight = \(\text{Kg} \times \text{m/s}^2\) This combination, \(\text{Kg} \cdot \text{m/s}^2\), is defined as the Newton (N). Step 5: Evaluate the given options
(1) Kg: This is the SI unit of mass, not weight.
(2) \(\text{Kg ms}^{-1}\): This is the SI unit of momentum (\(p = mv\)), not weight.
(3) N: This is the SI unit of force, and therefore, of weight.
(4) None of these: Incorrect, as (3) is the correct unit. Therefore, the SI unit of weight is the Newton (N).