Weight is a measure of the heaviness of an object. The term weight has two general meanings. In science and technology, the weight of an object is the gravitational force on the object. When weight is used in this sense, the units of weight are units of force. In the International System of Units (SI), the modern metric system, the unit of force is the newton. In the inch-pound system customarily used in the United States, units of force include the ounce and the pound. One pound equals 4.448 newtons.
In commercial and everyday use, weight is used to mean mass, the amount of matter in an object. When people use weight in this sense, they measure it in pounds or kilograms. The kilogram is the SI’s base unit of mass. One pound equals 0.454 kilogram.
The gravitational force on an object is related to the mass of the object by the equation F = mg, where F is the force, m is the mass, and g is the acceleration due to gravity. At the surface of the earth, g is about 9.8 meters per second per second. A one-kilogram object therefore has a weight—in the sense of gravitational force—of about 9.8 newtons (2.2 pounds). On the moon, where the acceleration due to gravity is about 1.6 meters per second per second, the object would still have a mass of one kilogram, but its weight would be about 1.6 newtons (0.4 pound).
