Weight is often measured in kilograms (kg) rather than Newtons (N) because the kilogram is a unit of mass, whereas the Newton is a unit of force.
Mass refers to the amount of matter an object contains, and it is a fundamental property of an object. Mass is measured in units like kilograms, grams, or pounds. The kilogram is the standard unit of mass in the International System of Units (SI).
Weight, on the other hand, is the force exerted on an object due to gravity. It is the result of the gravitational attraction between the object and the Earth (or another celestial body). Weight depends on both the mass of the object and the strength of the gravitational field it is in.
The unit for measuring force in the SI system is the Newton (N), named after Sir Isaac Newton. One Newton is defined as the force required to accelerate a one-kilogram mass by one meter per second squared (1 N = 1 kg·m/s²).
Weight can be calculated by multiplying an object's mass by the acceleration due to gravity. On or near the Earth's surface, the acceleration due to gravity is approximately 9.8 m/s². So, for example, an object with a mass of 10 kg would have a weight of approximately 98 N (10 kg × 9.8 m/s²).
While weight is a measure of force, it is common to use the unit of mass (kg) to refer to weight in everyday language, mainly due to historical convention and convenience. This is because the mass of an object doesn't change with the location or the strength of the gravitational field, whereas weight does. So, when people say something weighs 10 kg, they are technically referring to its mass, but it is often understood as its weight in everyday usage.