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...