inertia

inertia (în-ûr¹she), in physics, the resistance of a body to any alteration in its state of motion, i.e., the resistance of a body at rest to being set in motion or of a body in motion to any change of speed or of direction of motion. This is known as Newton's first law of motion.

mass

mass, in physics, the quantity of matter in a body regardless of its volume. There are two ways of measuring mass. Since weight is proportional to mass, the mass of a body may be determined using a balance and a set of standard masses (gravitational mass). You can also determine the mass using Newton's second law of motion by measuring the body's resistance to acceleration by an external force (m=F/a -->inertial mass). According to the special theory of relativity, mass increases with speed according to the formula m = m₀/(1-v²/c²)^1/2, where m₀ is the rest mass (mass at zero velocity) of the body, v its speed, and c the speed of light in vacuum. The theory also showed that mass and energy were equivalent and conversion from one to the other followed the Einstein mass-energy relation E = mc², where E is the energy and m the relativistic mass.