Mass vs Weight

Difference Between Mass and Weight

Students of physics often confuse mass and weight of an object and many also feel that there is no difference between the two, while the fact is that there is a lot of difference between the two.

Mass is the amount of matter present in a body and is an intrinsic property of the body. Mass of an object remains the same always at any place.

Weight on the other hand is the force which a given mass feels due to the gravity at its place. Weight is measured in units of Force like Newton (which is the SI unit of Force).

If your mass is 60 kgs then your weight is approximately 60 x 10 = 600 Newtons. This is because

Force = mass x acceleration (From Newton's second Law)

Thus, weight = mass x acceleration due to gravity

If you go to moon your mass remains same, i.e 60 kgs, but your weight becomes less by 1/6 amount, since moon's gravity is 1/6 that of earth.

Mass of a body is measured by balancing it equally with another known amount of mass. You keep known amount of masses like blocks of 1 kg, 2 kg etc on one side till both the sides balance and then add up the numbers on the known side of mass and thus calculate the unknown mass. This works because, when the masses are equal on both the sides of the balance the effect of gravity cancels out for both (i.e weight cancels out) and hence we can calculate the mass on one side of the balance if we know the mass on the other side of the balance.

Weight is measured using a scale which effectively measures the pull on the mass exerted by the gravity of the earth.

Table 1: Differences between Mass and Weight

	Mass	Weight
1.	Is always a constant at any place and time	Depends on gravity at the place
2.	Is measured in kilograms in SI unit	Is measured in Newtons (not in kilograms as one might think)
3.	Is measured using balance	Is measured using scales
4.	Can never be zero	Can also be zero
5.	Is an intrinsic property of a body and is independent of any external factor.	Depends on 1. Mass of the object which is attracting it 2. Force with which it is being attracted (which in turn depends on the distance between the two)

If an object is in freefall towards the attractor (like earth), even then it has weight, but it experiences weightlessness (like an astronaut in a spaceship around the earth) since it is obeying the force. Weight can be felt only when the body in question tends to oppose the force of gravity (like u and me sitting on the surface of the earth:-)

Remember that even though we are at rest due to the friction between our self and earth's surface, our acceleration is not zero, it is still 9.8 m/s² as the earth is constantly pulling us down towards its center. But we are resisting that pull and feel the force as weight.