Mass
Intuitively we know that mass is the amount of matter in an object, but physicists, especially Isaac Newton, very carefully defined mass as a body's resistance to being accelerated. By Newton's second law, F=ma, mass determines to what degree an object will respond to a specific force. For a given force the acceleration will be smaller if the mass is larger, and vice-versa.
Mass has inertia, or resistance to motion, and the mass in Newton's second law is called the inertial mass. Once in motion at a constant velocity, inertial mass resists changes to that motion. These observations are formalized in Newton's first law: "A body at rest will remain at rest and a body in motion will remain in motion with constant velocity unless an external net force acts upon the body."
Mass also responds to gravity. A dropped egg will be attracted to the Earth and crack when the floor stops it. Newton also derived the equation for this force, and of course it depends on an object's mass, which Newton called its gravitational mass. Much consideration was given to the question of whether the gravitational mass of a body was the same as its inertial mass? Experimentally it was found to be so. This was an important consideration in the development of the general theory of relativity by Albert Einstein around 1911.
Mass is measured in units of kilograms (kg). To define the kilogram, a cylinder of platinum- iridium alloy is used as the standard, and is kept at the International Bureau of Weights and Measures near Paris, France. Replicas of the standard mass are kept in various laboratories around the world.
In the early years of the 20th century, Einstein's special theory of relativity modified the concept of mass as it has been known since the time of Newton in the 17th century. Einstein imagined objects traveling very near the speed of light, and found that the inertial mass of a body increases as its energy increases. The energy increase can be of any type--an increase in kinetic energy of motion, an addition of heat resulting in a temperature rise, the energy released in a nuclear reaction--and is related to the increase in mass by Einstein's famous equation E=mc2. Another way to view this is that, as a body moves closer and closer to the speed of light, an observer trying to measure the mass finds that it increases with the body's velocity. Mass is thus more than the quantity of matter, Newton's original idea. It is in fact energy that can be converted into other types of energy such as heat, light such as radiation, or other bodies with mass. This fact has been critical to the developments in nuclear and particle physics in the twentieth century.
This is the complete article, containing 458 words
(approx. 2 pages at 300 words per page).
