What is the quantity used to measure weight?

Answer 1

By definition the weight of an object is the average force that the earth gravity exerts on the object on the earth surface. The average is to be taken into account due to the fact that earth gravity is not exactly constant over all the earth surface, but the deviation from average is small so that usually the weight is simply measured by measuring the gravity force in a single point on the earth surface (for example with a balance).

Being a force, the weight should be measured in Newtons (N), the measurement unit of Force in the international system. However, frequently an old measurement unit is used, coming from a now unused measurement unit system: the so called technical system.

This unit is the weight kilogram (weight kg or wkg) that is the force exerted by earth gravity on a body with a mass of one mass kilogram (simply indicated with kg).

Even if the use of the same name generates a lot of confusion, the weight kg has not to be confused with the mass kilogram, since they are measurement units of two different physical quantities: a force the first and a mass the second.

In terms of international system units, by definition a weight kg is equal to about 9.8 N, as it can be demonstrated by the Newton second principle

F = m g

where F is the force (that is the weight in this case), m is the mass (one kg mass for a weight of one kg weight) and g the gravity acceleration on the earth surface that is about 9.8 N/kg = 9.8 m/s2.

Answer 2

The quantity used to measure weight is mass, typically measured in units such as grams, kilograms, or pounds. Weight can vary depending on the force of gravity acting on an object, while mass is a constant measure of the amount of matter an object contains.