Weight vs. Mass
Overview
Mass (kg) is the amount of matter in an object — a scalar, constant everywhere. Weight W = mg is a gravitational force (N) — a vector pointing downward. On the Moon, your mass is the same but your weight is less (g_Moon ≈ 1.6 m/s²). Never write 'weight = 70 kg' — weight is in Newtons.
Mass: the amount of matter
Mass is a scalar quantity measured in kilograms (kg). It is a measure of the amount of matter in an object and also its inertia (resistance to acceleration). Mass is the same everywhere in the universe — your mass is 70 kg on Earth, on the Moon, in space. Mass does not change with location. It is measured with a balance (by comparing).
Weight: the gravitational force
Weight is the gravitational force acting on a mass: W = mg, where g is the gravitational field strength (m/s² = N/kg). Weight is a vector, directed toward the center of the gravitational body. Weight changes with location: on the Moon (g = 1.62 m/s²) you weigh about 1/6 of your Earth weight. Weight is measured with a spring scale (measures force). In everyday language, 'weight' is often misused to mean mass — in physics, they are distinct.
- ⚠Saying 'my weight is 70 kg' — weight is in Newtons, mass is in kg
- ⚠Using weight when you should use mass in F = ma (use mass in kg, not weight in N)
- ⚠Thinking weight is the same on all planets — weight depends on g, which varies