Mass
Mass is a fundamental scalar quantity that measures the amount of matter in an object and determines its resistance to acceleration (inertia). Unlike weight, mass does not depend on gravitational field strength and remains constant regardless of location in the universe. Mass is measured in kilograms (kg) in the SI system and plays a central role in Newton's Second Law, gravitational force calculations, and energy equations.
Mass vs Weight — Key Comparisons
| Property | Mass | Weight |
|---|---|---|
| Definition | Amount of matter | Gravitational force on object |
| Symbol | m | W or F_g |
| SI Unit | kilogram (kg) | Newton (N) |
| Scalar or Vector | Scalar | Vector |
| Changes with location? | No | Yes (depends on gravity) |
| Value on Moon (70 kg person) | 70 kg (unchanged) | ~114 N (≈ 1/6 of Earth weight) |
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Related Terms
Weight
Weight is the gravitational force exerted on an object due to a gravitational field, typically Earth's. It is a vector quantity directed toward the centre of the gravitational body and varies depending on the local gravitational acceleration. A person who weighs 686 N on Earth would weigh only about 114 N on the Moon, because the Moon's gravitational acceleration is approximately one-sixth that of Earth.
Inertia
Inertia is the tendency of an object to resist any change in its state of motion — whether at rest or moving at constant velocity. It is not a force but a property of matter that depends directly on the mass of the object; greater mass means greater inertia. Inertia explains why a heavy truck requires much more braking force than a bicycle to stop in the same distance, and why it is harder to start a loaded cart moving than an empty one.
Newton's Second Law
Newton's Second Law of Motion states that the net force acting on an object equals the product of its mass and acceleration. It is the most quantitative of the three laws and provides the mathematical relationship between force, mass, and motion. This law is used in virtually every engineering and physics calculation involving dynamics, from designing car brakes to launching spacecraft.
From Latin 'massa' meaning lump, dough, or bulk — itself from Greek 'maza' (barley cake). In physics, the term was used by Newton in 'Principia' (1687) to mean the 'quantity of matter'. The kilogram was defined as the SI base unit of mass at the first General Conference on Weights and Measures in 1889.