Mass
Mass is the measurement of inertia and inertia is the property of any object which opposes the change in the state of the object. It is inertia because of which an object at rest has tendency to remain at rest and an object in motion has tendency to remain in motion.
Inertia depends upon the mass of an object. The object having greater mass has greater inertia and vice versa. Mass of an object remains constant everywhere, i.e. mass will remain same whether that object is at the moon, at the earth or anywhere in the universe. Mass is a scalar quantity which has only magnitude but no direction. SI unit of mass is kilogram which is written in short form as kg. Mass of a body cannot be zero.
Weight
The force with which an object is attracted towards the centre of the earth is called the weight of the object. Thus, the weight is a force acting vertically downwards; it has both magnitude and direction.
Force, F = m × a
In case of free fall, a = g
So, F = m × g
But the force of attraction of the earth on an object is called its weight (W). So,
W = m × g
So, weight is the force and its SI unit is Newton (N). It depends on ‘g’ and is a vector quantity, which is acting vertically downwards.
Since the value of ‘g’ is approximate remains constant at or near the Earth’s surface, therefore, the expression ‘W = m x g’ can be written as follows:
W ∝ m
This means the weight of any object is directly proportional to its mass, i.e. weight will increase with the increase of mass and decrease with decrease in mass. This is the cause that the weight of any object is the measure of its mass.
The unit of weight
Since the weight of an object is equal to the force by which an object is attracted towards the earth; so, the unit of weight is same as the unit of force. Therefore, the unit of weight is Newton (N).
Relation between 1 kg wt and express it into Newton:
We know that, W = m × g
If mass (m) = 1 kg & g = 9.8 m/s^{2}, then
W = 1 kg × 9.8 m/s2
or, 1 kg wt = 9.8N
So, the gravitational force of the earth that acts on an object of mass 1 kg is called as 1 kg wt.
Comparison between Mass and Weight
Weight of an object on the Moon
We have learnt that the weight of an object on the earth is the force with which the earth attracts the object. In the same way, the weight of an object on the moon is the force with which the moon attracts that object. The mass of the moon is less than that of the earth. Due to this, the moon exerts a lesser force of attraction on objects. Weight of the object on the moon = (1/6) × its weight on the earth.