Weight

Weight is the force of gravity acting on a body when it is under gravitational influence, such as that of Earth. In classical physics, weight is a force and is therefore measured in newtons (N), while mass is measured in kilograms (kg).

The simplest formula is W = m·g, where m is the mass of the body and g is the gravitational acceleration. At Earth's surface, g is approximately 9.8 m/s² but varies slightly with altitude and latitude (the equator and poles differ).

Weight and mass

Mass is the amount of matter in a body, constant everywhere. Weight changes with gravitational field: a person with mass 70 kg weighs about 686 N on Earth, but would weigh roughly one-sixth of that on the Moon, because the Moon's gravity is weaker. In everyday speech, the word "weight" is often used for mass, measured in kilos—a distinction precise physics maintains.

Weightlessness and scales

A body in orbit around Earth experiences apparent "weightlessness" because it and the spacecraft fall together. Gravity still acts; what is absent is the support force from a surface. Similarly, a bathroom scale measures the normal force from contact, then converts to kilograms using standard g.

Use and measurement

Commerce, medicine, and engineering use different scales: a balance compares masses and so its reading does not change if g changes; a spring scale measures force and so depends on location. In the SI system, the base unit is kilograms for mass, with weight derived from it.