5.7 : Mass and Weight

Mass and weight are often used interchangeably in everyday conversation. For example, medical records often show our weight in kilograms, but never in the correct units of newtons. In physics, however, there is an important distinction. Weight is the pull of the Earth on an object. It depends on the distance from the center of the Earth. Weight dramatically varies if we leave the Earth's surface, unlike mass, which does not vary with location. On the Moon, for example, the acceleration due to gravity is only 1.67 m/s² compared to 9.8 m/s² on Earth. Therefore, a 1.0 kg mass has a weight of 9.8 N on the Earth, but only about 1.7 N on the Moon. However, the mass of an object is the same on the Earth's surface, in orbit, or on the surface of the Moon.

When an object is dropped, it accelerates toward the center of the Earth. Newton's second law states that a net force on an object is responsible for its acceleration. If air resistance is considered negligible, the net force acting on a falling object is equal to the gravitational force, commonly known as its weight, or the force due to gravity acting on an object of mass 'm'.

Note that, although they are closely related, weight and mass are different physical quantities. Mass is an intrinsic property of an object; it is a quantity of matter. It is tempting to equate mass to weight because most of our examples take place on the Earth, where the weight of an object varies minimally with the object's location on Earth. In addition, it is difficult to count and identify all of the atoms and molecules in an object, so mass is rarely determined in this manner.

This text is adapted from Openstax, University Physics Volume 1, Section 5.4: Mass and Weight.