Mass (physics)
Mass, in physics, the property of matter that causes it to have inertia. (Inertia is the resistance of an object to any change in its motion.) The more mass an object has, the more inertia it has. An older definition of mass—the amount of matter an object contains—is still used, but is not as useful to scientists because amounts of matter cannot be measured directly.
Mass is measured in such units as pounds and kilograms. These names are also commonly used for units of weight, but the units are not the same.
In the 19th century, scientists believed that mass cannot be created or destroyed. According to this principle, known as the law of conservation of mass, the mass of the ashes and gases produced when a piece of wood burns should exactly equal the original mass of the wood plus the mass of the oxygen that united with it.
Today scientists believe that mass is equivalent to energy and the total mass before and after a given process is exactly the same only if the energy gained or lost in the process is taken into account. The equivalence of mass and energy was deduced from the theory of relativity. The concept is of importance in calculations dealing with nuclear reactions and fast-moving particles. In everyday processes (such as the burning of wood) the amount of mass gained or lost as energy is insignificant and the law of conservation of mass is still considered to hold true.