People often say "weight" when they mean "mass" — and the confusion shows up in everyday conversions. A kilogram is technically a unit of mass, while a pound can be either mass or force depending on context. Understanding the distinction helps you convert correctly.
Mass vs Weight: The Physics
Mass is the amount of matter in an object — it does not change regardless of location. Weight is the force of gravity acting on that mass. On Earth, a 1 kg object weighs about 9.81 Newtons. On the Moon, the same object still has a mass of 1 kg but weighs only about 1.62 N.
The Kilogram: SI Base Unit
The kilogram is the SI base unit of mass. Since 2019, it is defined by the Planck constant (h = 6.626070159 × 10⁻³⁴ J·s). Common metric mass units: 1 tonne = 1,000 kg, 1 kg = 1,000 g, 1 g = 1,000 mg.
Pounds, Ounces, and Stones
The avoirdupois pound is defined as exactly 0.45359237 kilograms. One pound equals 16 ounces. The stone (14 pounds) is still used in the UK for body weight. Key conversions: 1 kg ≈ 2.205 lb, 1 lb ≈ 453.6 g, 1 oz ≈ 28.35 g.
Real-World References
A liter of water has a mass of approximately 1 kg. A US nickel weighs exactly 5 grams. A standard paperclip is about 1 gram. An average adult human has a mass of 60–80 kg (130–175 lb).
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