Generally, mass refers to how "heavy" something is. However, mass is really an "inertial" property; that is, the tendency of an object to remain at constant velocity unless acted upon by an outside force. According to Newton's second law of motion, as expressed in the formula an object with a mass, "m", of one kilogram will accelerate, "a", at one meter per second per second (about one-tenth the acceleration due to Earth's gravity) when acted upon by a force, "F", of one newton.