What is the difference between mass and weight?

Mass and weight are fundamentally different concepts often confused.

Mass: amount of matter; intrinsic property; constant regardless of location; SI unit: kilogram (kg); US: slug (rare) or pound-mass (lbm); measured with a BALANCE (compares to known masses).

Weight: gravitational force on mass; depends on gravity at the location; SI unit: Newton (N); US: pound-force (lbf); measured with a SCALE (measures force). Relationship: W = mg, where g = gravitational acceleration. On Earth's surface: g ≈ 9.8 m/s² (or 32 ft/s²). A 1 kg object weighs 9.8 N on Earth. A 1 lbm object weighs 1 lbf on Earth (because of how pound-mass is defined). Examples on other planets/moons: 70 kg person — on EARTH: weight = 70 × 9.8 = 686 N (about 154 lb); on MOON (g ≈ 1.6 m/s²): weight = 70 × 1.6 = 112 N (about 25 lb); on MARS (g ≈ 3.7 m/s²): weight = 70 × 3.7 = 259 N (about 58 lb); on JUPITER (g ≈ 24.8 m/s²): weight = 70 × 24.8 = 1736 N (about 390 lb); in deep SPACE (no gravity): weight ≈ 0, but mass still 70 kg. Everyday usage: 'I weigh 150 pounds' technically refers to weight; in physics precision, mass is 150 lbm. Bathroom scales measure weight but display as mass — assuming Earth gravity. Astronauts in orbit appear weightless because they're in free fall around Earth (the same as weightlessness in orbit — actually freefall, not absence of gravity). Mass affects: inertia (resistance to acceleration, F = ma); momentum (p = mv); kinetic energy (KE = ½mv²); gravitational attraction (between all masses). Weight affects: how hard it is to lift something against gravity; reading on a scale; gravitational potential energy in a gravity field.