Mass

Mass is a physical quantity that describes how much matter an object contains. In the International System of Units (SI), it is measured in kilograms. Unlike weight, which depends on the strength of gravity, mass remains constant wherever an object is. A person has the same mass on Earth, on the Moon or in space, even though their weight changes because gravity is different.

Mass is also a measure of inertia, the tendency of an object to resist changes in its motion. For this reason, pushing, stopping or accelerating a loaded truck requires much more force than doing the same with a bicycle. The greater the mass, the greater the force needed to change an object's speed or direction.

Mass is usually measured with a balance that compares an unknown mass with a known reference. Many kitchen scales and laboratory balances work according to this principle. Mass is one of the fundamental quantities in physics and appears in many important laws, including Newton's laws of motion and Einstein's famous equation E = mc², which shows that mass and energy are closely linked.

Although every object contains a large amount of energy in theory, only specific nuclear reactions can convert a small fraction of its mass into usable energy. Understanding mass helps explain how objects move, collide and interact, making it one of the foundations of modern physics.