Mass vs Weight
Mass and weight, perhaps are the most commonly used and misunderstood terms in daily life and in the study of physics . People often use weight when they mean mass, and vice versa. When we go to a vendor to buy tomatoes, we see them being weighed in kilograms. But to a physicist, it is wrong. It is true that the mass of the tomatoes is 1 kg, but its weight is the product of the gravity of the earth and it’s mass. The gravity of earth is taken to be 10N/kg, so the weight of 1kg of tomatoes would be 10N and not 1kg.
If we were to take our 1kg of tomatoes to the moon, though they still have the same mass, which is 1kg, their weight on moon would be much less because the gravity of moon is much less as compared to earth. But those who sell things on earth are not troubled by the concept of gravity as the gravity of earth is taken to be same everywhere. Still confused? Here is a clearer distinction between the two terms.
Mass is the measurement of the amount of matter contained by an object
Weight is the measurement of the force or pull of gravity on any object.
Mass is measured with a balance that compares a known amount with an unknown amount.
Weight is measured on a scale.
Mass of a body does not change if its location changes.
Weight of a body changes, depending upon the gravity of the location
Every body has a mass (say 1kg). It means it is heavy enough to weigh 1kg. Its weight depends upon how hard gravity is pulling it down. Since gravity is same everywhere on earth, its weight would be the same. But if the same object were to be taken to outer space, the weight would be 0, while the mass is still 1kg. This means that the mass of a body does not change but its weight can change. Similarly, on moon, which has gravity equal to 1/6th on earth, the same object would weigh 1/6 kg.
This means that the weight should not be expressed in kilograms. Indeed it is not, and the unit used to talk about weight is Newtons. But scales show weight in kilograms as people understand it better. If scales were to show weight in Newtons, people would be confused. So if your mass is 100kg, just multiply it by 9.8 (which is the gravity of earth) to arrive at your weight.